C++ Standard Core Language
Defect Reports,
Revision
49

This document contains the C++ core language issues that have been
categorized as Defect Reports by the Committee (J16 +
WG21), that is, issues with status
"DR,"
"WP," and
"TC1,"
along with their proposed resolutions. ONLY RESOLUTIONS FOR
ISSUES WITH TC1 STATUS ARE PART OF THE INTERNATIONAL STANDARD
FOR C++. The other issues are provided for informational
purposes only, as an indication of the intent of the Committee.
They should not be considered definitive until or unless they
appear in an approved Technical Corrigendum or revised
International Standard for C++.

This document is part of a group of related documents that
together describe the issues that have been raised regarding the
C++ Standard. The other documents in the group are:

Active Issues List, which contains
explanatory material for the entire document group and a list of
the issues that have not yet been acted upon by the Committee.

Closed Issues List, which contains
the issues which the Committee has decided are not defects
in the International Standard, including a brief rationale
explaining the reason for the decision.

Table of Contents, which contains a
summary listing of all issues in numerical order.

Index by Section, which contains a
summary listing of all issues arranged in the order of the
sections of the Standard with which they deal most directly.

Index by Status, which contains a
summary listing of all issues grouped by status.

For more information, including a description of the meaning of
the issue status codes and instructions on reporting new issues,
please see the Active Issues List.

Issues with "DR" Status

452.
Wording nit on description of this

9.3.2
class.this paragraph 1, which specifies the
meaning of the
keyword 'this', seems to limit its usage to the *body* of non-static
member functions. However 'this' is also usable in ctor-initializers
which, according to the grammar in
8.4
dcl.fct.def par. 1, are not
part of the body.

Proposed resolution: Changing the first part of
9.3.2
class.this par. 1 to:

In the body of a nonstatic (9.3) member function or in a
ctor-initializer (12.6.2), the keyword this is a non-lvalue
expression whose value is the address of the object for which
the function is called.

NOTE: I'm talking of constructors as functions that are "called";
there have been discussions on c.l.c++.m as to whether constructors
are "functions" and to whether this terminology is correct or not; I
think it is both intuitive and in agreement with the standard wording.

Steve Adamczyk: See also issue 397,
which is defining a new syntax term for the body of a function
including the ctor-initializers.

The potential scope of a name declared in a class consists not only of
the declarative region following the name's point of declaration, but
also of all function bodies,bodies and default
arguments, and constructor ctor-initializers in that
class (including such things in nested classes).

The potential scope of a declaration that extends to or past the end
of a class definition also extends to the regions defined by its
member definitions, even if the members are defined lexically outside
the class (this includes static data member definitions, nested class
definitions, member function definitions (including the member
function body and, for constructor functions (12.1
class.ctor), the ctor-initializer (12.6.2
class.base.init))
and any portion of the declarator part of such definitions which
follows the identifier, including a parameter-declaration-clause
and any default arguments (8.3.6
dcl.fct.default).
[Example:...

That is, an unqualified name that occurs, for instance, in a type or
default argument expression in the parameter-declaration-clause,parameter-declaration-clause or in the function body,
or in an expression of a mem-initializer in a constructor
definition.

...Within the class member-specification, the class is regarded as
complete within function bodies, default arguments, andexception-specifications, and constructor
ctor-initializers (including such things in nested classes)...

Each occurrence in an expression of the name of a non-static data
member or non-static member function of a class shall be expressed as
a class member access (5.2.5
expr.ref), except when it
appears in the formation of a pointer to member (5.3.1
expr.unary.op), oror when it appears in the body of
a non-static member function of its class or of a class derived from
its class (9.3.1
class.mfct.nonstatic), or when it appears in
a mem-initializer for a constructor for its class or for a
class derived from its class (12.6.2
class.base.init).

[Note: a name used in a member function definition (that is, in the
parameter-declaration-clause including the default arguments
(8.3.6
dcl.fct.default), oror in the member
function body, or, for a constructor function (12.1
class.ctor), in a mem-initializer expression (12.6.2
class.base.init)) is looked up as described in 3.4
basic.lookup. —end note]

...A non-static member function may also be called directly using
the function call syntax (5.2.2
expr.call, 13.3.1.1
over.match.call) from within the body of a member function of its class or of a class derived from its class.

from within the body of a member function of its class or of a
class derived from its class, or

from a mem-initializer (12.6.2
class.base.init) for
a constructor for its class or for a class derived from its class.

When an id-expression (5.1
expr.prim) that is not
part of a class member access syntax (5.2.5
expr.ref)
and not used to form a pointer to member (5.3.1
expr.unary.op) is used in the body of a non-static member function
of class Xor used in the mem-initializer for a
constructor of class X, if name lookup (3.4.1
basic.lookup.unqual) resolves the name in the id-expression to a
non-static non-type member of class X or of a base class
of X, the id-expression is transformed into a class
member access expression (5.2.5
expr.ref)
using (*this) (9.3.2
class.this) as the
postfix-expression to the left of the . operator...

...The implicitly-defined default constructor performs the set of
initializations of the class that would be performed by a user-written
default constructor for that class with an
empty mem-initializer-listno ctor-initializer (12.6.2
class.base.init)
and an empty function bodycompound-statement...

...After the call to a constructor for class X has completed, if a
member of X is neither specified in the constructor’s
mem-initializers, nor default-initialized, nor
value-initialized, nor given a value during execution
of the compound-statement of the body of the
constructor, the member has indeterminate value.

A function-try-block associates a handler-seq with the
ctor-initializer, if present, and the function-bodycompound-statement. An exception thrown during the
execution of the initializer expressions in
the ctor-initializer or during the execution of
the function-bodycompound-statement
transfers control to a handler in a
function-try-block in the same way as an exception thrown
during the execution of a try-block transfers control to other
handlers. [Example:

When an exception is thrown, control is transferred to the nearest
handler with a matching type (15.3
except.handle);
“nearest” means the handler for which the
compound-statement,compound-statement
orctor-initializer, or function-body following
the try keyword was most recently entered by the thread of
control and not yet exited.

Issues with "WP" Status

357.
Definition of signature should include name

Section 1.3.10
defns.signature, definition of "signature"
omits the function name as part of the signature. Since the name participates
in overload resolution, shouldn't it be included in the definition?
I didn't find a definition of signature in the ARM,
but I might have missed it.

Fergus Henderson:
I think so. In particular, 17.4.3.1.2
lib.global.names
reserves certain "function
signatures" for use by the implementation, which would be wrong
unless the signature includes the name.

-2- Each global function signature declared with external linkage in a
header is reserved to the implementation to designate that function
signature with external linkage.

-5- Each function signature from the Standard C library declared with
external linkage is reserved to the implementation for use as a
function signature with both extern "C" and extern "C++" linkage,
or as a name of namespace scope in the global namespace.

Other uses of the term "function signature" in the description of the
standard library also seem to assume that it includes the name.

James Widman:

Names don't participate in overload resolution; name lookup is
separate from overload resolution. However, the word
“signature” is not used in clause 13
over.
It is used in linkage and declaration matching (e.g.,
14.5.5.1
temp.over.link). This suggests that the name
and scope of the function should be part of its signature.

the name and the parameter-type-list (8.3.5
dcl.fct)
of a function, as well as the class or namespace of which it is a
member. If a function or function template is a class member its
signature additionally includes the cv-qualifiers (if any) on
the function or function template itself. The signature of a function
template additionally includes its return type and its template
parameter list. The signature of a function template specialization
includes the signature of the template of which it is a specialization
and its template arguments (whether explicitly specified or deduced).
[Note: Signatures are used as a basis for name-mangling and
linking. —end note]

Delete paragraph 3 and replace the first sentence of
14.5.5.1
temp.over.link as follows:

The signature of a function template specialization consists of the
signature of the function template and of the actual template
arguments (whether explicitly specified or deduced).

The signature of a function template consists of its function
signature, its return type and its template parameter listis
defined in 1.3.10
defns.signature. The names
of the template parameters are significant...

537.
Definition of “signature”

The standard defines “signature” in two places:
1.3.10
defns.signature and 14.5.5.1
temp.over.link
paragraphs 3-4. The former seems to be meant as a formal definition
(I think it's the only place covering the nontemplate case), yet it
lacks some bits mentioned in the latter (specifically, the notion of a
“signature of a function template,” which is part of every
signature of the associated function template specializations).

Also, I think the 1.3.10
defns.signature words “the
information about a function that participates in overload
resolution” isn't quite right either. Perhaps, “the
information about a function that distinguishes it in a set of
overloaded functions?”

Eric Gufford:

In 1.3.10
defns.signature the definition states that
“Function signatures do not include return type, because that
does not participate in overload resolution,” while 14.5.5.1
temp.over.link paragraph 4 states “The signature of a
function template consists of its function signature, its return type
and its template parameter list.” This seems inconsistent and
potentially confusing. It also seems to imply that two identical
function templates with different return types are distinct
signatures, which is in direct violation of 13.3
over.match. 14.5.5.1
temp.over.link paragraph 4 should be
amended to include verbiage relating to overload resolution.

Either return types are included in function signatures, or they're
not, across the board. IMHO, they should be included as they are an
integral part of the function declaration/definition irrespective of
overloads. Then verbiage should be added about overload resolution to
distinguish between signatures and overload rules. This would help
clarify things, as it is commonly understood that overload
resolution is based on function signature.

In short, the term “function signature” should be made
consistent, and removed from its (implicit, explicit or otherwise)
linkage to overload resolution as it is commonly understood.

James Widman:

The problem is that (a) if you say the return type is part of the
signature of a non-template function, then you have overloading but
not overload resolution on return types (i.e., what we have now with
function templates). I don't think anyone wants to make the language
uglier in that way. And (b) if you say that the return type is not
part of the signature of a function template, you will break code.
Given those alternatives, it's probably best to maintain the status
quo (which the implementors appear to have rendered faithfully).

513.
Non-class “most-derived” objects

The standard uses “most derived object” in some places
(for example, 1.3.3
defns.dynamic.type, 5.3.5
expr.delete) to refer to objects of both class and non-class
type. However, 1.8
intro.object only formally defines it for
objects of class type.

If a complete object, a data member (9.2
class.mem), or
an array element is of class type, its type is considered the most
derived class, to distinguish it from the class type of any base
class subobject; an object of a most derived class type, or of a
non-class type, is called a most derived object.

362.
Order of initialization in instantiation units

Should this program do what its author obviously expects? As far as
I can tell, the standard says that the point of instantiation for
Fib<n-1>::Value is the same as the point of instantiation as the
enclosing specialization, i.e., Fib<n>::Value. What in the standard
actually says that these things get initialized in the right order?

John Spicer:
My opinion is that the standard does not specify
the behavior of this program.
I thought there was a core issue related to this, but I could not find it.
The issue that I recall proposed tightening up the static initialization
rules to make more cases well defined.

Your comment about point of instantiation is correct, but I don't think that
really matters. What matters is the order of execution of the initialization
code at execution time. Instantiations don't really live in
"translation units"
according to the standard. They live in "instantiation units", and the
handling of instantiation units in initialization is unspecified (which should
probably be another core issue). See 2.1
lex.phases
paragraph 8.

Notes from October 2002 meeting:

We discussed this and agreed that we really do mean the the order
is unspecified. John Spicer will propose wording on handling of
instantiation units in initialization.

Objects with static storage duration defined in namespace scope in the
same translation unit and dynamically initialized shall be initialized
in the order in which their definition appears in the translation
unit.

Dynamic initialization of an object is either ordered or unordered.
Explicit specializations and definitions
of class template static data members have ordered
initialization. Other class template static data member instances have
unordered initialization. Other objects defined in namespace scope
have ordered initialization. Objects defined within a single
translation unit and with ordered initialization shall be initialized
in the order of their definitions in the translation unit. The order
of initialization is unspecified for objects with unordered
initialization and for objects defined in different translation units.

This addresses this issue but while reviewing this issue some
additional changes were suggested for the above wording:

Dynamic initialization of an object is either ordered or unordered.
Definitions of explicitly specializedExplicit specializations and definitions of
class template static data members have ordered
initialization. Other class template static data members (i.e.,
implicitly or explicitly instantiated specializations)instances have
unordered initialization. Other objects defined in namespace scope
have ordered initialization. Objects defined within a single
translation unit and with ordered initialization shall be initialized
in the order of their definitions in the translation unit. The order
of initialization is unspecified for objects with unordered
initialization and for objects defined in different translation units.

505.
Conditionally-supported behavior for unknown character escapes

If the character following a backslash is not one of those specified,
the behavior is undefined.

Paper J16/04-0167=WG21 N1727 suggests that such character escapes
be ill-formed. In discussions at the Lillehammer meeting, however,
the CWG felt that the newly-approved category of
conditionally-supported behavior would be more appropriate.

261.
When is a deallocation function "used?"

3.2
basic.def.odr paragraph 2 says that a deallocation
function is "used" by a new-expression or
delete-expression appearing in a potentially-evaluated
expression. 3.2
basic.def.odr paragraph 3 requires only that
"used" functions be defined.

This wording runs afoul of the typical implementation technique for
polymorphic delete-expressions in which the deallocation
function is invoked from the virtual destructor of the most-derived
class. The problem is that the destructor must be defined, because
it's virtual, and if it contains an implicit reference to the
deallocation function, the deallocation function must also be defined,
even if there are no relevant new-expressions or
delete-expressions in the program.

Is it required that D::operator delete(void*) be defined,
even if no B or D objects are ever created or
deleted?

Suggested resolution: Add the words "or if it is found by
the lookup at the point of definition of a virtual destructor
(12.4
class.dtor)" to the specification in 3.2
basic.def.odr paragraph 2.

Notes from 04/01 meeting:

The consensus was in favor of requiring that any declared
non-placement operator delete member function be
defined if the destructor for the class is defined (whether
virtual or not), and similarly for a non-placement
operator new if a constructor is defined.

An allocation or deallocation function for a class is
used by a new expression appearing in a potentially-evaluated
expression as specified in 5.3.4
expr.new
and 12.5
class.free.
A deallocation function for a class is used by a delete expression
appearing in a potentially-evaluated expression as specified in
5.3.5
expr.delete and 12.5
class.free.
A non-placement allocation or deallocation function for a
class is used by the definition of a constructor of that class. A
non-placement deallocation function for a class is used by the
definition of the destructor of that class, or by being selected by
the lookup at
the point of definition of a virtual destructor (12.4
class.dtor).
[Footnote: An implementation is not required to call
allocation and deallocation functions from constructors or
destructors; however, this is a permissible implementation
technique.]

Does (1) introduce U as a visible name in the surrounding
namespace scope?

If not, then (2) could presumably be an error since the
"union U" in that definition does not find the same type
as the declaration (1).

If yes, then (3) is OK too. However, we have gone through
much trouble to allow template implementations that do not
pre-parse the template definitions, but requiring (1) to
be visible would change that.

if the elaborated-type-specifier is used in the
decl-specifier-seq or parameter-declaration-clause
of a function defined in namespace scope, the identifier is declared
as a class-name in the namespace that contains the
declaration; otherwise, except as a friend declaration, the identifier
is declared in the smallest non-class, non-function-prototype scope
that contains the declaration. [Note: These rules also apply within
templates.] [Note: ...]

Most (all?) compilers reject this code because
D is handled as a template name rather than as
the injected class name.

9
class/2 says that the injected class
name is "inserted into the scope of the class."

3.3.6
basic.scope.class/1 seems to be the text
intended to describe
what "scope of a class" means, but it assumes that
every name in that scope was introduced using a
"declarator". For an implicit declaration such
as the injected-class name it is not clear what
that means.

So my questions:

Should the injected class name be available
in the base class specifiers?

John Spicer:
I do not believe the injected class name should be available in the base
specifier. I think the semantics of injected class names should be as
if a magic declaration were inserted after the opening "{" of the class
definition. The injected class name is a member of the class and
members don't exist at the point where the base specifiers are scanned.

Do you agree the wording should be clarified
whatever the answer to the first question?

John Spicer:
I believe the 3.3.6
basic.scope.class wording should be
updated to reflect the fact that not all names come from declarators.

1) The potential scope of a name declared in a class consists not only of the
declarative region following the name's point of declarationdeclarator,
but also of all function
bodies, default arguments, and constructor ctor-initializers
in that class (including such things in nested classes).
The point of declaration of an injected-class-name
(clause 9
class) is
immediately following the opening brace of the class definition.

The question here is whether the lookup for i in the
initializer of N::j finds the declaration in namespace
N or not. Implementations differ on this question.

If N::j were a static data member of a class, the
answer would be clear: both 3.4.1
basic.lookup.unqual
paragraph 12 and 8.5
dcl.init paragraph 11 say that
the initializer “is in the scope of the member's
class.” There is no such provision for namespace
members defined outside the namespace, however.

A name used in the definition of a static data member of
class X (9.4.2
class.static.data) (after the
qualified-id of the static member) is looked up as if the name
was used in a member function of X.

It is certainly the case that a name used in a function that is
a member of a namespace is looked up in that namespace
(3.4.1
basic.lookup.unqual paragraph 6), regardless of whether
the definition is inside or outside that namespace. Initializers
for namespace members should probably be looked up the same way.

If a variable member of a namespace is defined outside of the scope
of its namespace then any name used in the definition of the variable
member (after the declarator-id) is looked up as if the
definition of the variable member occurred in its
namespace. [Example:

143.
Friends and Koenig lookup

When an unqualified name is used as the postfix-expression
in a function call
(5.2.2
expr.call
)... namespace-scope
friend function declarations
(11.4
class.friend
) not otherwise
visible may be found... the set of declarations found
by the lookup of the function name [includes] the set
of declarations found in the... classes associated with
the argument types.

The most straightforward reading of this wording is that if a function of
namespace scope (as opposed to a class member function) is
declared as a friend in a class, and that class is an
associated class in a function call, the friend function will
be part of the overload set, even if it is not visible to
normal lookup.

This example would seem to satisfy the criteria from
3.4.2
basic.lookup.argdep
:
A::S is an associated class of the argument, and
A::S has a
friend declaration of the namespace-scope function
B::f(A::S),
so Koenig lookup should include B::f(A::S) as part of the
overload set in the call.

Another interpretation is that, instead of finding the friend
declarations in associated classes, one only looks for namespace-scope
functions, visible or invisible, in the namespaces of which the the
associated classes are members; the only use of the friend
declarations in the associated classes is to validate whether an
invisible function declaration came from an associated class or not
and thus whether it should be included in the overload set or not.
By this interpretation, the call f(s) in the example will
fail, because B::f(A::S) is not a member of namespace
A and thus is not found by the lookup.

Notes from 10/99 meeting: The second interpretation
is correct. The wording should be revised to make clear that Koenig
lookup works by finding "invisible" declarations in namespace scope
and not by finding friend declarations in associated
classes.

Proposed resolution (04/01): The "associated classes"
are handled adequately under this interpretation by
3.4.2
basic.lookup.argdep paragraph 3, which describes the
lookup in the associated namespaces as including the friend
declarations from the associated classes. Other mentions of the
associated classes should be removed or qualified to avoid the
impression that there is a lookup in those classes:

When an unqualified name is used as the postfix-expression
in a function call (5.2.2
expr.call), other
namespaces not considered during the usual unqualified lookup
(3.4.1
basic.lookup.unqual) may be searched, and
namespace-scope friend function declarations
(11.4
class.friend) not otherwise visible may be found.

to

When an unqualified name is used as the postfix-expression
in a function call (5.2.2
expr.call), other
namespaces not considered during the usual unqualified lookup
(3.4.1
basic.lookup.unqual) may be searched, and
in those namespaces,
namespace-scope friend function declarations
(11.4
class.friend) not otherwise visible may be found.

In 3.4.2
basic.lookup.argdep paragraph 2, delete
the words and classes in the following two sentences:

If the ordinary unqualified lookup of the name finds the
declaration of a class member function, the associated namespaces
and classes are not considered. Otherwise the
set of declarations found by the lookup of the function name is
the union of the set of declarations found using ordinary
unqualified lookup and the set of declarations found in the
namespaces and classes associated with the
argument types.

218.
Specification of Koenig lookup

The original intent of the Committee when Koenig lookup was added
to the language was apparently something like the following:

The name in the function call expression is looked up like any
other unqualified name.

If the ordinary unqualified lookup finds nothing or finds the
declaration of a (non-member) function, function template, or overload
set, argument-dependent lookup is done and any functions found in
associated namespaces are added to the result of the ordinary
lookup.

This approach is not reflected in the current wording of the
Standard. Instead, the following appears to be the status quo:

Lookup of an unqualified name used as the
postfix-expression in the function call syntax always performs
Koenig lookup (3.4.1
basic.lookup.unqual paragraph 3).

Unless ordinary lookup finds a class member function, the result
of Koenig lookup always includes the declarations found in associated
namespaces (3.4.2
basic.lookup.argdep paragraph 2), regardless of
whether ordinary lookup finds a declaration and, if so, what kind of
entity is found.

The declarations from associated namespaces are not limited to
functions and template functions by anything in 3.4.2
basic.lookup.argdep. However, if Koenig lookup results in more than one
declaration and at least one of the declarations is a non-function,
the program is ill-formed (7.3.4
namespace.udir, paragraph 4;
although this restriction is in the description of the
using-directive, the wording applies to any lookup that spans
namespaces).

John Spicer: Argument-dependent lookup was created to solve
the problem of looking up function names within templates where you
don't know which namespace to use because it may depend on the
template argument types (and was then expanded to permit use in
nontemplates). The original intent only concerned functions. The
safest and simplest change is to simply clarify the existing wording
to that effect.

Bill Gibbons: I see no reason why non-function declarations
should not be found. It would take a special rule to exclude
"function objects", as well as pointers to functions, from
consideration. There is no such rule in the standard and I see no
need for one.

If the ordinary unqualified lookup of the name finds the declaration
of a class member function, the associated namespaces and classes are
not considered.

This implies that if the ordinary lookup of the name finds the
declaration of a data member which is a pointer to function or
function object, argument-dependent lookup is still done.

My guess is that this is a mistake based on the incorrect
assumption that finding any member other than a member function would
be an error. I would just change "class member function" to "class
member" in the quoted sentence.

Mike Miller: In light of the issue of "short-circuiting"
Koenig lookup when normal lookup finds a non-function, perhaps it
should be written as "...finds the declaration of a class member, an
object, or a reference, the associated namespaces..."?

Andy Koenig: I think I have to weigh in on the side of
extending argument-dependent lookup to include function objects and
pointers to functions. I am particularly concerned about [function
objects], because I think that programmers should be able to replace
functions by function objects without changing the behavior of their
programs in fundamental ways.

Bjarne Stroustrup: I don't think we could seriously argue
from first principles that [argument-dependent lookup should find only
function declarations]. In general, C++ name lookup is designed to be
independent of type: First we find the name(s), then, we consider
its(their) meaning. 3.4
basic.lookup states "The name
lookup rules apply uniformly to all names ..." That is an important
principle.

Thus, I consider text that speaks of "function call" instead of
plain "call" or "application of ()" in the context of koenig lookup an
accident of history. I find it hard to understand how 5.2.2
expr.call doesn't either disallow all occurrences of
x(y) where x is a class object (that's clearly not
intended) or requires koenig lookup for x independently of
its type (by reference from 3.4
basic.lookup). I suspect
that a clarification of 5.2.2
expr.call to mention
function objects is in order. If the left-hand operand of () is a
name, it should be looked up using koenig lookup.

John Spicer: This approach causes otherwise well-formed
programs to be ill-formed, and it does so by making names visible that
might be completely unknown to the author of the program.
Using-directives already do this, but argument-dependent lookup is
different. You only get names from using-directives if you actually
use using-directives. You get names from argument-dependent
lookup whether you want them or not.

This basically breaks an important reason for having namespaces.
You are not supposed to need any knowledge of the names used by a
namespace.

But this example breaks if argument-dependent lookup finds
non-functions and if the translation unit includes the <list>
header somewhere.

This really makes namespaces of questionable value if you still
need to avoid using the same name as an entity in another namespace to
avoid problems like this.

Erwin Unruh: Before we really decide on this topic, we should
have more analysis on the impact on programs. I would also like to see
a paper on the possibility to overload functions with function
surrogates (no, I won't write one). Since such an extension is bound
to wait until the next official update, we should not preclude any
outcome of the discussion.

I would like to have a change right now, which leaves open several
outcomes later. I would like to say that:

Koenig lookup will find non-functions as well. If it finds a variable, the
program is ill-formed.
If the primary lookup finds a variable, Koenig lookup is done. If the result
contains both functions and variables, the program is ill-formed.
[Note: A future standard will assign semantics to such a program.]

I myself are not comfortable with this as a long-time result, but it
prepares the ground for any of the following long term solutions:

Do overloading on mixed function/variable sets.

Ignore variables on Koenig lookup.

Don't do Koenig lookup if the primary lookup finds a variable.

Find variables on Koenig lookup and give an error if there is a
variable/function mix.

The note is there to prevent compiler vendors to put their own
extensions in here.

Although many agreed that there were valid concerns motivating a
desire for Koenig lookup to find non-function declarations, there was
also concern that supporting this capability would be more dangerous
than helpful in the absence of overload resolution for mixed function
and non-function declarations.

A straw poll of the group revealed 8 in favor of Koenig lookup
finding functions and function templates only, while 3 supported the
broader result.

Notes from the 10/01 meeting:

There was unanimous agreement on one less controversial point:
if the normal lookup of the identifier finds a non-function, argument-dependent
lookup should not be done.

On the larger issue, the primary point of consensus is that making
this change is
an extension, and therefore it should wait until the point at
which we are considering extensions (which could be very soon).
There was also consensus on the fact that the standard as it
stands is not clear: some introductory text suggests that
argument-dependent lookup finds only functions, but the more
detailed text that describes the lookup does not have any
such restriction.

It was also noted that some existing implementations (e.g., g++) do
find some non-functions in some cases.

The issue at this point is whether we should (1) make a small change
to make the standard clear (presumably in the direction of not
finding the non-functions in the lookup), and revisit the issue
later as an extension, or (2) leave the standard alone for now and
make any changes only as part of considering the extension.
A straw vote favored option (1) by a strong majority.

Additional Notes (September, 2006):

Recent discussion of this issue
has emphasized the following points:

The concept of finding function pointers and function objects
as part of argument-dependent lookup is not currently under active
discussion in the Evolution Working Group.

The major area of concern with argument-dependent lookup is
finding functions in unintended namespaces. There are current
proposals to deal with this concern either by changing the
definition of “associated namespace” so that fewer
namespaces are considered or to provide a mechanism for enabling
or disabling ADL altogether. Although this concern is conceptually
distinct from the question of whether ADL finds function pointers
and function objects, it is related in the sense that the current
rules are perceived as finding too many functions (because of
searching too many namespaces), and allowing function pointers
and function objects would also increase the number of entities
found by ADL.

Any expansion of ADL to include function pointers and
function objects must necessarily update the overloading rules
to specify how they interact with functions and function
templates in the overload set. Current implementation experience
(g++) is not helpful in making this decision because, although
it performs a uniform lookup and finds non-function entities, it
diagnoses an error in overload resolution if non-function entities
are in the overload set.

There is a possible problem if types are found by ADL: it
is not clear that overloading between callable entities
(functions, function templates, function pointers, and function
objects) and types (where the postfix syntax means a cast or
construction of a temporary) is reasonable or useful.

James Widman:

There is a larger debate here about whether ADL should find object
names; the proposed wording below is only intended to answer the
request for wording to clarify the status quo (option 1 above) and
not to suggest the outcome of the larger debate.

Proposed Resolution (October, 2006):

Replace the normative text in 3.4.2
basic.lookup.argdep
paragraph 3 with the following (leaving the text
of the note and example unchanged):

Let X be the lookup set produced by unqualified lookup
(3.4.1
basic.lookup.unqual) and let Y be the lookup set
produced by argument dependent lookup (defined as follows). If X
contains

a declaration of a class member, or

a block-scope function declaration that is not
a using-declaration, or

a declaration that is neither a function nor a function template

then Y is empty. Otherwise Y is the set of declarations
found in the namespaces associated with the argument types as
described below. The set of declarations found by the lookup of
the name is the union of X and Y.

When considering an associated namespace, the lookup is the same
as the lookup performed when the associated namespace is used as
a qualifier (3.4.3.2
namespace.qual) except that:

Any using-directives in the associated namespace are
ignored.

Any namespace-scope friend functions or friend function
templates declared in associated classes are visible within their
respective namespaces even if they are not visible during an
ordinary lookup (11.4
class.friend).

All names except those of (possibly overloaded) functions
and function templates are ignored.

403.
Reference to a type as a template-id

If T is a template-id, its associated namespaces and classes are the
namespace in which the template is defined; for member templates,
the member template's class; the namespaces and classes associated
with the types of the template arguments provided for template type
parameters (excluding template template parameters); the namespaces
in which any template template arguments are defined; and the
classes in which any member templates used as template template
arguments are defined. [Note: non-type template arguments do not
contribute to the set of associated namespaces. ]

There is a problem with the term "is a template-id". template-id
is a syntactic construct and you can't really talk about a type being a
template-id. Presumably, this is intended to mean "If T is the type of a
class template specialization ...".

557.
Does argument-dependent lookup cause template instantiation?

One might assume from 14.7.1
temp.inst paragraph 1
that argument-dependent lookup would require instantiation of any
class template specializations used in argument types:

Unless a class template specialization has been explicitly
instantiated (14.7.2
temp.explicit) or explicitly specialized
(14.7.3
temp.expl.spec), the class template specialization is
implicitly instantiated when the specialization is referenced in a
context that requires a completely-defined object type or when the
completeness of the class type affects the semantics of the program.

A complete class type is required to determine the associated
classes and namespaces for the argument type (to determine the class's
bases) and to determine the friend functions declared by the class, so
the completeness of the class type certainly “affects the
semantics of the program.”

If T is a class type (including unions), its associated
classes are: the class itself; the class of which it is a member, if
any; and its direct and indirect base classes. Its associated
namespaces are the namespaces in which its associated classes are
defined.

A class template specialization is a class type, so the second
bullet would appear to apply, requiring the specialization to be
instantiated in order to determine its base classes.

However, bullet 8 of that paragraph deals explicitly with class
template specializations:

If T is a class template specialization its
associated namespaces and classes are the namespace in which the
template is defined; for member templates, the member template’s
class; the namespaces and classes associated with the types of the
template arguments provided for template type parameters (excluding
template template parameters); the namespaces in which any template
template arguments are defined; and the classes in which any member
templates used as template template arguments are
defined.

Note that the class template specialization itself is not
listed as an associated class, unlike other class types, and there is
no mention of base classes. If bullet 8 were intended as a supplement
to the treatment of class types in bullet 2, one would expect phrasing
along the lines of, “In addition to the associated namespaces
and classes for all class types...” or some such; instead,
bullet 8 reads like a self-contained and complete specification.

If argument-dependent lookup does not cause implicit
instantiation, however, examples like the following fail:

If T is a class type (including unions), its associated
classes are: the class itself; the class of which it is a member, if
any; and its direct and indirect base classes. Its associated
namespaces are the namespaces inof which its associated
classes are definedmembers. Furthermore,
if T is a class template specialization, its associated
namespaces and classes also include: the namespaces and classes
associated with the types of the template arguments provided for
template type parameters (excluding template template parameters); the
namespaces of which any template template arguments are members; and
the classes of which any member templates used as template template
arguments are members. [Note: Non-type template arguments do not
contribute to the set of associated namespaces. —end
note]

If T is a class template specialization its associated
namespaces and classes are the namespace in which the template is
defined; for member templates, the member template’s class; the
namespaces and classes associated with the types of the template
arguments provided for template type parameters (excluding template
template parameters); the namespaces in which any template template
arguments are defined; and the classes in which any member templates
used as template template arguments are defined. [Note: non-type
template arguments do not contribute to the set of associated
namespaces. —end note]

If the nested-name-specifier of a qualified-id nominates
a class, the
name specified after the nested-name-specifier is looked up in the
scope of the class (10.2
class.member.lookup), except for the cases
listed below. The name shall represent one or more members of that
class or of one of its base classes (clause 10
class.derived).
If the class-or-namespace-name of the nested-name-specifier
names a cv-qualified class type, it nominates the underlying class
(the cv-qualifiers are ignored).

Notes from 4/02 meeting:

There is a problem in that class-or-namespace-name does not
include typedef names for cv-qualified class types. See
7.1.3
dcl.typedef paragraph 4:

Here's a good question: in this example, should X
be used as a name-for-linkage-purposes (FLP name)?

typedef class { } const X;

Because a type-qualifier is parsed as a
decl-specifier, it isn't possible to declare cv-qualified
and cv-unqualified typedefs for a type in a single declaration.
Also, of course, there's no way to declare a typedef for the
cv-unqualified version of a type for which only a cv-qualified version
has a name.
So, in the above example, if X isn't used as the FLP name,
then there can be no FLP name.
Also note that a FLP name usually represents a parameter type, where
top-level cv-qualifiers are usually irrelevant anyway.

Data points: for the above example, Microsoft uses X as the
FLP name; GNU and EDG do not.

My recommendation: for consistency with the direction we're going on
this issue, for simplicity of description (e.g., "the first
class-name declared by the declaration"), and for (very
slightly) increased utility, I think Microsoft has this right.

If the typedef declaration defines an unnamed class type (or
enum type), the first typedef-name declared by the
declaration to behave that class type
(or enum type)or a cv-qualified version thereof
is used to denote the class type (or enum type) for linkage purposes
only (3.5
basic.link).
[Example: ...

This is only a note, and it is at least incomplete (and quite possibly
inaccurate), despite (or because of) its complexity.
I propose to delete it.

...
[Note: a typedef-name that names a class is a
class-name (9.1
class.name).
Except as the identifier in the declarator for a constructor
or destructor definition outside of a class
member-specification (12.1
class.ctor,
12.4
class.dtor), a typedef-name
that names a class may be used in a qualified-id to refer to
a constructor or destructor. ]

My first choice would have been to make this the primary statement about
the equivalence of typedef-name and class-name,
since the equivalence comes about as a result of a typedef declaration.
Unfortunately, references to class-name point to
9.1
class.name, so it
would seem that the primary statement should be there instead.
To avoid the possiblity of conflicts in the future, I propose to make
this a note.

[Note:
A typedef-name that names a class type, or a cv-qualified
version thereof, is also a class-name
(9.1
class.name).
If a typedef-name is used following the
class-key in an elaborated-type-specifier
(7.1.5.3
dcl.type.elab),
or in the class-head of a class declaration (9
class),
or is used as the identifier in the declarator for a
constructor or destructor declaration (12.1
class.ctor,
12.4
class.dtor),to identify the subject of an elaborated-type-specifier
(7.1.5.3
dcl.type.elab),
class declaration (clause 9
class),
constructor declaration (12.1
class.ctor),
or destructor declaration (12.4
class.dtor),
the program is ill-formed.
]
[Example: ...

When I looked carefully into the statement of the rule prohibiting a
typedef-name in a constructor declaration, it appeared to me
that this grammar rule (inadvertently?) allows something that's always
forbidden semantically.

Unlike the prohibitions against appearing in an
elaborated-type-specifier or constructor or destructor
declarator, each of which was expressed more than once, the prohibition
against a typedef-name appearing in a class-head
was previously stated only in 7.1.3
dcl.typedef.
It seems to me that that prohibition belongs here instead.
Also, it seems to me important to clarify that a typedef-name
that is a class-name is still a typedef-name.
Otherwise, the various prohibitions can be argued around easily, if
perversely ("But that
isn't a typedef-name, it's a class-name; it says
so right there in 9.1
class.name.")

A typedef-name (7.1.3
dcl.typedef)
that names a class type or a
cv-qualified version thereof is
also a class-name, but shall not be used
in an elaborated-type-specifier; see also
7.1.3
dcl.typedef.as the identifier in a class-head.

The new nonterminal references are needed to really nail down what we're
talking about here.
Otherwise, I'm just eliminating redundancy.
(A typedef-name that doesn't name a class type is no more
valid here than one that does.)

A typedef-name that names a class is a
class-name (7.1.3
dcl.typedef); however, aAtypedef-namethat names a class shall not be
used as the identifierclass-name in the declaratordeclarator-id for a constructor declaration.

...
A typedef-name that names a class is a
class-name (7.1.3); however, aAtypedef-namethat names a class shall not be
used as the identifierclass-name
following the ~ in the declarator for a destructor
declaration.

318.
struct A::A should not name the constructor of A

A use of an injected-class-name in an elaborated-type-specifier
should not name the constructor of the class, but rather the class itself,
because in that context we know that we're looking for a type.
See issue 147.

If the nested-name-specifier nominates a class C, and the name
specified after the nested-name-specifier, when looked up in C, is the
injected class name of C (clause 9
class), the name is instead
considered to name the constructor of class C.

with

In a lookup in which the constructor is an acceptable lookup result,
if the nested-name-specifier nominates a class C and the name
specified after the nested-name-specifier, when looked up in C, is the
injected class name of C (clause 9
class), the name is
instead considered to name the constructor of class C.
[Note: For example, the constructor is not an acceptable lookup result in an
elaborated type specifier so the constructor would not be used in place
of the injected class name.]

I can't find anything in the standard that says definitively what this
means. 7.3.3
namespace.udecl says that a using-declaration shall
name "a member of a base class" -- but here we have two members, the
data member A::i and the class A::i.

Personally, I'd find it more attractive if this code did not work. I'd
like "using A::i" to mean "lookup A::i in the usual way and bind B::i to
that", which would mean that while "i = 3" would be valid in D::f,
"struct i x" would not be. However, if there were no A::i data member,
then "A::i" would find the struct and the code in D::f would be valid.

John Spicer:
I agree with you, but unfortunately the standard committee did not.

I remembered that this was discussed by the committee and that a
resolution was adopted that was different than what I hoped for, but I
had a hard time finding definitive wording in the standard.

I went back though my records and found the paper that proposed a
resolution and the associated committee motion that adopted the
proposed resolution The paper is N0905, and "option 1" from that paper
was adopted at the Stockholm meeting in July of 1996. The resolution
is that "using A::i" brings in everything named i from A.

3.4.3.2
namespace.qual paragraph 2 was modified to
implement this resolution, but interestingly
that only covers the namespace case and not the class case. I think
the class case was overlooked when the wording was drafted. A core
issue should be opened to make sure the class case is handled
properly.

A
using-declaration
used as a
member-declaration
shall refer to a member of a base class of the class being defined,
shall refer to a member of an anonymous union that is a member of a base class
of the class being defined, or
shall refer to an enumerator for an enumeration type that is a member of a base
class of the class being defined.

to

In a using-declaration used as a
member-declaration, the nested-name-specifier
shall name a base class of the class being defined. Such a
using-declaration introduces the set of declarations found by member
name lookup (10.2
class.member.lookup,
3.4.3.1
class.qual).

245.
Name lookup in elaborated-type-specifiers

I have some concerns with the description of name lookup for
elaborated type specifiers in 3.4.4
basic.lookup.elab:

Paragraph 2 has some parodoxical statements concerning looking up
names that are simple identifers:

If the elaborated-type-specifier refers to an enum-name
and this lookup does not find a previously declared enum-name,
the elaborated-type-specifier is ill-formed. If the
elaborated-type-specifier refers to an [sic]class-name and this lookup does not find a previously declared
class-name... the elaborated-type-specifier is a
declaration that introduces the class-name as described in
3.3.1
basic.scope.pdecl."

It is not clear how an elaborated-type-specifier can refer
to an enum-name or class-name given that the lookup does
not find such a name and that class-name and enum-name
are not part of the syntax of an elaborated-type-specifier.

The second sentence quoted above seems to suggest that the name found
will not be used if it is not a class name. typedef-name names
are ill-formed due to the sentence preceding the quote. If lookup
finds, for instance, an enum-name then a new declaration will
be created. This differs from C, and from the enum case, and can have
surprising effects:

Was this really the intent? If this is the case then some more
work is needed on 3.4.4
basic.lookup.elab. Note that the
section does not make finding a type template formal ill-formed, as is
done in 7.1.5.3
dcl.type.elab. I don't see anything that
makes a type template formal name a class-name. So the example
in 7.1.5.3
dcl.type.elab of friend class T; where
T is a template type formal would no longer be ill-formed
with this interpretation because it would declare a new class
T.

As given in N1376=02-0034. Note that the inserts and strikeouts in
that document do not display
correctly in all browsers; <del> --> <strike>
and <ins> --> <b>, and the
similar changes for the closing delimiters,
seem to do the trick.

254.
Definitional problems with elaborated-type-specifiers

The text in 3.4.4
basic.lookup.elab paragraph 2 twice
refers to the possibility that an elaborated-type-specifier
might have the form

class-key identifier;

However, the grammar for elaborated-type-specifier does
not include a semicolon.

In both 3.4.4
basic.lookup.elab and
7.1.5.3
dcl.type.elab, the text asserts that an
elaborated-type-specifier that refers to a typedef-name
is ill-formed. However, it is permissible for the form of
elaborated-type-specifier that begins with typename
to refer to a typedef-name.

This problem is the result of adding the typename form
to the elaborated-type-name grammar without changing the
verbiage correspondingly. It could be fixed either by updating the
verbiage or by moving the typename syntax into its own
production and referring to both nonterminals when needed.

(See also issue 180. If this
issue is resolved in favor of a separate nonterminal in the
grammar for the typename forms, the wording in that
issue's resolution must be changed accordingly.)

Notes from 04/01 meeting:

The consensus was in favor of moving the typename
forms out of the elaborated-type-specifier grammar.

The problem is that 3.4.5
basic.lookup.classref says that you have to look
up A in both the context of the pointed-to-type (i.e.,
::A), and
in the context of the postfix-expression (i.e., the body of C::f), and
that if the name is found in both places it must name the same type in
both places.

The EDG front end does not issue an error about this program, though.

Am I reading the standardese incorrectly?

John Spicer: I think you are reading it correctly. I think
I've been hoping that this would
get changed. Unlike other dual lookup contexts, this is one in which the
compiler already knows the right answer (the type must match that of the left
hand of the -> operator). So I think that if either of the types
found matches
the one required, it should be sufficient. You can't say a->~::A(),
which
means you are forced to say a->::A::~A(), which disables the virtual
mechanism. So you would have to do something like create a local typedef
for the desired type.

If the id-expression in a class member access (5.2.5
expr.ref) is an unqualified-id, and the type of the
object expression is of a class type C(or of pointer to a
class type C), the unqualified-id is looked up in
the scope of class C...

If the unqualified-id is ~type-name,
the type-name is looked up in the context of the entire
postfix-expression.andIf the
type T of the object expression is of a class
type C(or of pointer to a class type C),
the type-name is also looked up in the context of the
entire postfix-expression and in the scope of
class C. The type-name shall refer to
a class-name. If type-name is found in both contexts,
the name shall refer to the same class type. If the type of the object
expression is of scalar type, the type-name is looked up in the
scope of the complete postfix-expression.At least one
of the lookups shall find a name that refers to (possibly
cv-qualified)
T. [Example:

381.
Incorrect example of base class member lookup

The example in 3.4.5
basic.lookup.classref paragraph 4
is wrong (see 11.2
class.access.base paragraph 5; the cast to the
naming class can't be done) and needs to be corrected.
This was noted when the final version of the algorithm for
issue 39 was checked against it.

Proposed Resolution (October 2003):

Remove the entire note at the end of
3.4.5
basic.lookup.classref paragraph 4, including the
entire example.

414.
Multiple types found on destructor lookup

By 3.4.5
basic.lookup.classref paragraph 3, the following is
ill-formed because the two lookups of the destructor name (in
the scope of the class of the object and in the surrounding
context) find different Xs:

struct X {};
int main() {
X x;
struct X {};
x.~X(); // Error?
}

This is silly, because the compiler knows what the type has
to be, and one of the things found matches that. The lookup
should require only that one of the lookups finds the required
class type.

In addition, a member function, static data member, class or
enumeration of class scope has external linkage if the name of the
class has external linkage.

to:

In addition, a member function, a static data member, a named class or
enumeration of class scope, or an unnamed class or enumeration defined
in a class-scope typedef declaration such that the class or
enumeration has the typedef name for linkage purposes
(7.1.3
dcl.typedef),
has external linkage if the name of the class has external linkage.

Paragraph 8 of Section 3.5
basic.link contains the following
sentences: "A name with no linkage shall not be used to declare an
entity with linkage. If a declaration uses a typedef name, it is the
linkage of the type name to which the typedef refers that is
considered."

The problem with this wording is that it doesn't cover cases where the
type to which a typedef-name refers has no name. As a result it's not
clear whether, for example, the following program is well-formed:

My feeling is that the rules for whether or not a typedef-name used in
a declaration shall be treated as having or not having linkage ought
to be modelled after those for dependent types, which are explained in
14.6.2.1
temp.dep.type.

Add the following text at the end of Paragraph 8 of Section
3.5
basic.link and replace the following example:

In case of the type referred to by a typedef declaration not having a name,
the newly declared typedef-name has linkage if and only if its referred type
comprises no names of no linkage excluding local names that are eligible for
appearance in an integral constant-expression (5.19
expr.const).
[Note: if the referred
type contains a typedef-name that does not denote an unnamed class, the
linkage of that name is established by the recursive application of this
rule for the purposes of using typedef names in declarations.] [Example:

Paragraph 2 of Section 14.3.1
temp.arg.type is inaccurate and
unnecessarily prohibits a few important cases; it says "A local type, a type
with no linkage, an unnamed type or a type compounded from any of these
types shall not be used as a template-argument for a template-parameter."
The inaccuracy stems from the fact that it is not a type but its name that
can have a linkage.

For example based on the current wording of 14.3.1
temp.arg.type,
the following example is ill-formed.

Replace the whole second paragraph of Section 14.3.1
temp.arg.type
with the following wording:

A type whose name does not have a linkage or a type compounded from any such
type shall not be used as a template-argument for a template-parameter. In
case of a type T used as a template type argument not having a name,
T
constitutes a valid template type argument if and only if the name of an
invented typedef declaration referring to T would have linkage;
see 3.5.
[Example:

389.
Unnamed types in entities with linkage

A name with no linkage (notably, the name of a class or enumeration
declared in a local scope (3.3.2
basic.scope.local)) shall not
be used to declare an entity with linkage.
If a declaration uses a typedef name, it is the linkage of the type
name to which the typedef refers that is considered.

The problem here is that most implementations must generate the same
mangled name
for "f" in two translation units. The quote from the standard above
isn't quite
clear, unfortunately: There is no type name to which the typedef refers.

A related situation is the following:

enum { no, yes } answer;

The variable "answer" is declared as having external linkage, but it is
declared
with an unnamed type. Section 3.5
basic.link
talks about the linkage of names, however,
and does therefore not prohibit this. There is no implementation issue
for most
compilers because they do not ordinarily mangle variable names, but I
believe
the intent was to allow that implementation technique.

Finally, these problems are much less relevant when declaring names
with internal
linkage. For example, I would expect there to be few problems with:

typedef struct {} *UP;
static void g(UP);

I recently tried to interpret 3.5
basic.link paragraph 8
with the assumption that types with no names
have no linkage. Surprisingly, this resulted in many diagnostics on
variable declarations (mostly like "answer" above).

I'm pretty sure the standard needs clarifying words in this matter, but
which way should it go?

There was agreement that this check is not needed for variables
and functions with extern "C" linkage, and a change there is
desirable to allow use of legacy C headers. The check is also not needed for
entities with internal linkage, but there was no strong sentiment
for changing that case.

We also considered relaxing this requirement for extern "C++"
variables but decided that we did not want to change that case.

We noted that if extern "C" functions are allowed an additional
check is needed when such functions are used as arguments in calls
of function templates. Deduction will put the type of the extern "C"
function into the type of the template instance, i.e., there would
be a need to mangle the name of an unnamed type. To plug that hole
we need an additional requirement on the template created in
such a case.

A name with no linkage (notably, the name of a class or enumeration
declared in a local scope (3.3.2
basic.scope.local))
shall not be used to declare an
entity with linkage. If a declaration uses a typedef name, it is
the linkage of the type name to which the typedef refers that is
considered.

to

A type is said to have linkage if and only if

it is a class or enumeration type that is named (or has a name for
linkage purposes (7.1.3
dcl.typedef))
and the name has linkage; or

it is a specialization of a class template (14
temp)
[Footnote: a class template always has external linkage, and
the requirements of 14.3.1
temp.arg.type
and 14.3.2
temp.arg.nontype
ensure that the template arguments will also have appropriate linkage]; or

A type without linkage shall not be used as the type of a variable or
function with linkage, unless the variable or function has extern "C"
linkage (7.5
dcl.link).
[Note: in other words, a type without linkage contains
a class or enumeration that cannot be named outside
of its translation unit. An entity with external linkage
declared using such a type could not correspond to any other entity
in another translation unit of the program and is thus not permitted.
Also note that classes with linkage may contain members whose types do
not have linkage, and that typedef names are ignored in the determination of
whether a type has linkage.]

When a block scope declaration of an entity with linkage is not
found to refer to some other declaration, then that entity is a
member of the innermost enclosing namespace.

The question then is whether N is an “enclosing
namespace” for the local declaration of g()?

Proposed resolution (October 2004):

Add the following text as a new paragraph at the end of
7.3.1
namespace.def:

The enclosing namespaces of a declaration are those
namespaces in which the declaration lexically appears, except for
a redeclaration of a namespace member outside its original
namespace (e.g., a definition as specified in 7.3.1.2
namespace.memdef). Such a redeclaration has the same enclosing
namespaces as the original declaration. [Example:

Initialization of static data members of class templates shall be
performed during the initialization of static data members for the
first translation unit to have static initialization performed for
which the template member has been instantiated. This requirement
shall apply to both the static and dynamic phases of initialization.

Notes from 04/01 meeting:

Enforcing an order of initialization on static data members of
class templates will result in substantial overhead on access to such
variables. The problem is that the initialization be required as the
result of instantiation in a function used in the initialization of a
variable in another translation unit. In current systems, the order of
initialization of static data data members of class templates is not
predictable. The proposed resolution is to state that the order of
initialization is undefined.

Objects with static storage duration defined in namespace scope in the
same translation unit and dynamically initialized shall be initialized
in the order in which their definition appears in the translation
unit.

with

Dynamic initialization of an object is either ordered or unordered.
Explicit specializations and definitions
of class template static data members have ordered
initialization. Other class template static data member instances have
unordered initialization. Other objects defined in namespace scope
have ordered initialization. Objects defined within a single
translation unit and with ordered initialization shall be initialized
in the order of their definitions in the translation unit. The order
of initialization is unspecified for objects with unordered
initialization and for objects defined in different translation units.

The sentinel list node is used to represent "nil" (the null pointer
cannot be used with my implementation, for reasons which are immaterial
to this discussion). All of the List's non-static member functions and
constructors depend upon the value of the sentinel. Under the proposed
resolution for issue #270, Lists cannot be safely instantiated before
main() begins, as the sentinel's initialization is "unordered".

(Some readers may propose that I should use the "singleton pattern" in the
List class. This is undesirable, for reasons I shall describe at the
end of this post at the location marked "[*]". For the moment, indulge
me by assuming that "singleton" is not an adequate solution.)

Though this is a particular example from my own experience, I believe it
is representative of a general class of examples. It is common to use
static data members of a class to represent the "distinguished values"
which are important to instances of that class. It is imperative that
these values be initialized before any instances of the class are
created, as the instances depend on the values.

In a comp.std.c++ posting on 28 Jul 2001, Brian McNamara proposes
the following alternative resolution:

Objects with static storage duration defined in namespace scope in
the same translation unit and dynamically initialized shall be
initialized in the order in which their definition appears in the
translation unit.

with

Objects with static storage duration defined in namespace scope
shall be initialized in the order described below.

and then after paragraph 1, add this text:

Dynamic initialization is either ordered or quasi-ordered. Explicit
specializations of class template static data members have ordered
initialization. Other class template static data member instances have
quasi-ordered initialization. All other objects defined in namespace
scope have ordered initialization. The order of initialization is
specified as follows:

Objects that are defined within a single translation unit and
that have ordered initialization shall be initialized in the
order of their definitions in the translation unit.

Objects that are defined only within a single translation unit
and that have quasi-ordered initialization shall also be
initialized in the order of their definitions in the translation
unit -- that is, as though these objects had ordered initialization.

Objects that are defined within multiple translation units (which,
therefore, must have quasi-ordered initialization) shall be
initialized as follows: in exactly one translation unit
(which one is unspecified), the object shall be treated as
though it has ordered initialization; in the other translation
units which define the object, the object will be initialized
before all other objects that have ordered initialization in
those translation units.

For any two objects, "X" and "Y", with static storage duration
and defined in namespace scope, if the previous bullets do not
imply a relationship for the initialization ordering between "X"
and "Y", then the relative initialization order of these objects
is unspecified.

along with a non-normative note along the lines of

[ Note: The intention is that translation units can each be compiled
separately with no knowledge of what objects may be re-defined in
other translation units. Each translation unit can contain a method
which initializes all objects (both quasi-ordered and ordered) as
though they were ordered. When these translation units are linked
together to create an executable program, all of these objects can
be initialized by simply calling the initialization methods (one
from each translation unit) in any order. Quasi-ordered objects
require some kind of guard to ensure that they are not initialized
more than once (the first attempt to initialize such an object
should succeed; any subsequent attempts should simply be ignored). ]

Erwin Unruh replies:
There is a point which is not mentioned with this posting.
It is the cost for implementing the scheme. It requires that each
static template variable is instantiated in ALL translation units
where it is used. There has to be a flag for each of these variables
and this flag has to be checked in each TU where the instantiation
took place.

I would reject this idea and stand with the proposed resolution of
issue 270.

There just is no portable way to ensure the "right" ordering of
construction.

Notes from 10/01 meeting:

The Core Working Group reaffirmed its previous decision.

441.
Ordering of static reference initialization

I have a couple of questions about 3.6.2
basic.start.init,
"Initialization of
non-local objects." I believe I recall some discussion of
related topics, but I can't find anything relevant in the issues
list.

The first question arose when I discovered that different
implementations treat reference initialization differently.
Consider, for example, the following (namespace-scope) code:

Because "int&" is not a POD type, there is no requirement that it
be initialized before dynamic initialization is performed, and
implementations differ in this regard. Using a function called
during dynamic initialization to print the values of "ip" and
"&ir", I found that g++, Sun, HP, and Intel compilers initialize
ir before dynamic initialization and the Microsoft compiler does
not. All initialize ip before dynamic initialization. I believe
this is conforming (albeit inconvenient :-) behavior.

So, my first question is whether it is intentional that a
reference of static duration, initialized with a reference
constant expression, need not be initialized before dynamic
initialization takes place, and if so, why?

The second question is somewhat broader. As
3.6.2
basic.start.init is currently
worded, it appears that there are no requirements on when ir is
initialized. In fact, there is a whole category of objects --
non-POD objects initialized with a constant expression -- for
which no ordering is specified. Because they are categorized as
part of "static initialization," they are not subject to the
requirement that they "shall be initialized in the order in which
their definition appears in the translation unit." Because they
are not POD types, they are not required to be initialized before
dynamic initialization occurs. Am I reading this right?

My preference would be to change
3.6.2
basic.start.init paragraph 1 so that 1) references
are treated like POD objects with respect to initialization, and
2) "static initialization" applies only to POD objects and
references. Here's some sample wording to illustrate:

Suggested resolution:

Objects with static storage duration (3.7.1) shall be
zero-initialized (8.5) before any other initialization takes
place. Initializing a reference, or an object of POD
type, of static storage duration with a constant expression
(5.19) is called constant initialization. Together,
zero-initialization and constant initialization are called
static initialization; all other initialization is dynamic
initialization. Static initialization shall be performed
before any dynamic initialization takes place. [Remainder
unchanged.]

Objects with static storage duration (3.7.1) shall be
zero-initialized (8.5) before any other initialization takes
place. Initializing a reference, or an object of POD type, of
static storage duration with a constant expression (5.19) is
called constant initialization. Together,
zero-initialization and constant initialization are
Zero-initialization and initialization with a
constant expression are collectively called static
initialization; all other initialization is dynamic
initialization. Static initialization shall be performed
Objects of POD types (3.9) with static storage
duration initialized with constant expressions (5.19) shall be
initialized before any dynamic initialization takes
place.

Any other allocation function that fails to allocate storage shall
only indicate failure by throwing an exception of
class std::bad_alloc (18.4.2.1
lib.bad.alloc) or a
class derived from std::bad_alloc.

Shouldn't this statement have the usual requirements for an
unambiguous and accessible base class?

Any other allocation function that fails to allocate storage shall
only indicate failure only by throwing an exception of
class std::bad_alloc (18.4.2.1
lib.bad.alloc) or a
class derived from std::bad_alloca type that would
match a handler (15.3
except.handle) of type
std::bad_alloc (18.4.2.1
lib.bad.alloc).

348.
delete and user-written deallocation functions

Standard is clear on behaviour of default allocation/deallocation
functions.
However, it is surpisingly vague on requirements to the behaviour
of user-defined deallocation function and an interaction between
delete-expression and deallocation function.
This caused a heated argument on fido7.su.c-cpp newsgroup.

Resume:

It is not clear if user-supplied deallocation function is called from
delete-expr when the operand of delete-expr is the null
pointer (5.3.5
expr.delete).
If it is, standard does not specify what user-supplied
deallocation function shall do with
the null pointer operand (18.4.1
lib.new.delete).
Instead, Standard uses the term "has no effect", which meaning
is too vague in context given (5.3.5
expr.delete).

Argument passed to delete-expression is valid - it is the result
of a call to the non-throwing version of ::new, which has been failed.
5.3.5
expr.delete paragraph 1 explicitly prohibit us to pass 0
without having the ::new failure.

Standard does NOT specify whether user-defined deallocation function
should be called in this case, or not.

...if the value of the operand of delete is the null pointer the operation
has no effect.

Standard doesn't specify term "has no effect".
It is not clear from this context, whether the called deallocation function
is required to have no effect, or delete-expression shall not call
the deallocation function.

We believe that study of 18.4.1.1
lib.new.delete.single paragraphs
12 and 13, 18.4.1.2
lib.new.delete.array paragraphs 11 and 12, and
3.7.3.2
basic.stc.dynamic.deallocation paragraph 3 shows that the
system-provided operator delete functions must accept a null pointer and
ignore it. Those sections also show that a user-written replacement
for the system-provided operator delete functions must accept a
null pointer. There is no requirement that such functions ignore
a null pointer, which is okay -- perhaps the reason for replacing the
system-provided functions is to do something special
with null pointer values (e.g., log such calls and return).

We believe that the standard should not require an implementation
to call a delete function with a null pointer, but it must allow
that. For the system-provided delete functions or replacements
thereof, the standard already makes it clear that the delete
function must accept a null pointer. For class-specific delete
functions, we believe the standard should require that such
functions accept a null pointer, though it should not mandate
what they do with null pointers.

5.3.5
expr.delete needs to be updated to say that
it is unspecified whether or not the operator delete function is
called with a null pointer, and 3.7.3.2
basic.stc.dynamic.deallocation
needs to be updated to say that any deallocation function must
accept a null pointer.

If the operand has a class type, the operand is converted to a
pointer type by calling the above-mentioned conversion function,
and the converted operand is used in place of the original
operand for the remainder of this section. In either alternative,
if the value of the operand of deleteis the
null pointer the operation has no effectmay be a null
pointer value. If it is not a null pointer value, inIn the first alternative (delete object), the value
of the operand of delete shall be a pointer to a
non-array object or a pointer to a sub-object (1.8
intro.object) representing a base class of such an object
(clause 10
class.derived)...

Change 5.3.5
expr.delete paragraph 4 as
follows (note that the old wording reflects the changes proposed
by issue 442:

The cast-expression in a delete-expression shall
be evaluated exactly once. If the delete-expression calls the
implementation deallocation function (3.7.3.2
basic.stc.dynamic.deallocation), and if the value of the operand of the
delete expression is not a null pointer, the deallocation
function will deallocate the storage referenced by the pointer
thus rendering the pointer invalid. [Note: the value of a
pointer that refers to deallocated storage is
indeterminate. —end note]

TheIf the value of the operand of the
delete-expression is not a null pointer value, thedelete-expression will invoke the destructor (if any) for
the object or the elements of the array being deleted. In the
case of an array, the elements will be destroyed in order of
decreasing address (that is, in reverse order of the completion
of their constructor; see 12.6.2
class.base.init).

TheIf the value of the operand of the delete-expression is not a
null pointer value, thedelete-expression will call a
deallocation function (3.7.3.2
basic.stc.dynamic.deallocation). Otherwise, it is unspecified
whether the deallocation function will be called. [Note: The
deallocation function is called regardless of whether the
destructor for the object or some element of the array throws an
exception. —end note]

The value of the first argument supplied to one of thea
deallocation functions provided in the standard library may be a
null pointer value; if so, and if the deallocation function is
one supplied in the standard library, the call to the
deallocation function has no effect. Otherwise, the value
supplied to operator delete(void*) in the standard library shall
be one of the values returned by a previous invocation of either
operator new(std::size_t) or operator new(std::size_t, const
std::nothrow_t&) in the standard library, and the value supplied
to operator delete[](void*) in the standard library shall be one
of the values returned by a previous invocation of either
operator new[](std::size_t) or operator new[](std::size_t, const
std::nothrow_t&) in the standard library.

In the code above, class S does have a non-trivial constructor, the
default constructor generated by the compiler. According the text
above, the lifetime of the auto s would never begin because a
constructor for S is never called. I think the second case in the
text needs to include aggregate initialization.

Mike Miller:
I see a couple of ways of fixing the problem. One way would be
to change "the constructor call has completed" to "the object's
initialization is complete."

Another would be to add following "a class type with a non-trivial
constructor" the phrase "that is not initialized with the brace
notation (8.5.1
dcl.init.aggr
)."

The first formulation treats aggregate initialization like a
constructor call; even POD-type members of an aggregate could
not be accessed before the aggregate initialization completed.
The second is less restrictive; the POD-type members of the
aggregate would be usable before the initialization, and the
members with non-trivial constructors (the only way an
aggregate can acquire a non-trivial constructor) would be
protected by recursive application of the lifetime rule.

If T is a class type with a non-trivial constructor
(12.1
class.ctor), the constructor call has
completed.

to

If T is a class type with a non-trivial constructor
(12.1
class.ctor), the initialization is complete.
[Note: the initialization can be performed by a
constructor call or, in the case of an aggregate with an
implicitly-declared non-trivial default constructor, an
aggregate initialization (8.5.1
dcl.init.aggr).]

The wording in 3.8
basic.life paragraph 6 allows an
lvalue designating an out-of-lifetime object to be used as the
operand of a static_cast only if the conversion is
ultimately to "char&" or "unsigned char&".
This description excludes the possibility of using a cv-qualified
version of these types for no apparent reason.

158.
Aliasing and qualification conversions

3.10
basic.lval paragraph 15 lists the types via which
an lvalue can be used to access the stored value of an object; using
an lvalue type that is not listed results in undefined behavior. It
is permitted to add cv-qualification to the actual type of the object
in this access, but only at the top level of the type ("a cv-qualified
version of the dynamic type of the object").

However, 4.4
conv.qual paragraph 4 permits a
"conversion [to] add cv-qualifiers at levels other than the first in
multi-level pointers." The combination of these two rules
allows creation of pointers that cannot be dereferenced without
causing undefined behavior. For instance:

int* jp;
const int * const * p1 = &jp;
*p1; // undefined behavior!

The reason that *p1 results in undefined behavior
is that the type of the lvalue is const int * const",
which is not "a cv-qualified version of" int*.

Since the conversion is permitted, we must give it defined
semantics, hence we need to fix the wording in 3.10
basic.lval to include all possible conversions of the type via
4.4
conv.qual.

Proposed resolution (04/01):

Add a new bullet to 3.10
basic.lval paragraph 15,
following "a cv-qualified version of the dynamic type of the
object:"

A type similar (as defined in 4.4
conv.qual) to
the dynamic type of the object,

An rvalue of type “pointer to cvT,” where
T is an object type, can be converted to an rvalue of type “pointer to
cvvoid”. The result of converting a “pointer to
cvT” to a “pointer
to cvvoid” points to the start of the storage
location where the object of type T resides, as if the object
is a most derived object (1.8
intro.object) of
type T (that is, not a base class subobject). The null
pointer value is converted to the null pointer value of the
destination type.

An rvalue of type “pointer to cv1void”
can be converted to an rvalue of type “pointer
to cv2T,” where T is an object type
and cv2 is the same cv-qualification as, or greater
cv-qualification than, cv1. The null pointer value is
converted to the null pointer value of the destination type. A
value of type pointer to object converted to “pointer
to cvvoid” and back, possibly with different
cv-qualification, shall have its original value...

480.
Is a base of a virtual base also virtual?

When the Standard refers to a virtual base class, it should be
understood to include base classes of virtual bases. However,
the Standard doesn't actually say this anywhere, so when
4.11
conv.mem (for example) forbids casting to a
derived class member pointer from a virtual base class member
pointer, it could be read as meaning:

...If B is an inaccessible (clause 11
class.access),
ambiguous (10.2
class.member.lookup) or virtual (10.1
class.mi) base class of D, or a base class of a
virtual base class of D, a program that necessitates this
conversion is ill-formed...

351.
Sequence point error: unspecified or undefined?

Between the previous and next sequence point a scalar object shall
have its stored value modified at most once by the evaluation of an
expression. Furthermore, the prior value shall be accessed only to
determine the value to be stored. The requirements of this
paragraph shall be met for each allowable ordering of the
subexpressions of a full expression; otherwise the behavior is
undefined. Example:

i = v[i++]; // the behavior is unspecified
i = 7, i++, i++; // i becomes 9
i = ++i + 1; // the behavior is unspecified
i = i + 1; // the value of i is incremented

451.
Expressions with invalid results and ill-formedness

If during the evaluation
of an expression, the result is not mathematically defined or not in
the range of representable values for its type, the behavior is
undefined, unless such an expression is a constant expression (5.19),
in which case the program is ill-formed.

Well, we do know that except in some contexts (e.g. controlling
expression of a #if, array bounds), a compiler is not required to
evaluate constant-expressions in compile time, right?

Now, let us consider, the following simple snippet:

if (a && 1/0)
...

with a, to fix our attention, being *not* a constant expression. The
quote above seems to say that since 1/0 is a constant
(sub-)expression, the program is ill-formed. So, is it the intent that
such ill-formedness is diagnosable at run-time? Or is it the intent
that the above gives undefined behavior (if 1/0 is evaluated) and is
not ill-formed?

I think the intent is actually the latter, so I propose the following
rewording of the quoted section:

If an expression is evaluated but its result is not mathematically
defined or not in the range of representable values
for its type the behavior is undefined, unless such an expression
is a constant expression (5.19) that shall be evaluated during
program translation, in which case the program is ill-formed.

Rationale (March, 2004):

We feel the standard is clear enough. The quoted sentence
does begin "If during the evaluation of an expression, ..."
so the rest of the sentence does not apply to an expression
that is not evaluated.

Note (September, 2004):

Gennaro Prota feels that the CWG missed the point of his
original comment: unless a constant expression appears in a
context that requires a constant expression, an
implementation is permitted to defer its evaluation to runtime.
An evaluation that fails at runtime cannot affect the
well-formedness of the program; only expressions that are
evaluated at compile time can make a program ill-formed.

If during the evaluation of an expression, the result is not
mathematically defined or not in the range of representable
values for its type, the behavior is undefined, unless such an
expression is a constant expressionappears where an
integral constant expression is required (5.19
expr.const), in which case the program is ill-formed.

What uses of template-ids
as unqualified-ids is this supposed to prevent? And is the
list of referenced sections correct/complete? For instance,
what about 14.8.1
temp.arg.explicit,
"Explicit template argument specification?"
Does its absence from the list in
5.1
expr.prim paragraph 11 mean that
"f<int>()" is ill-formed?

This is even more confusing when you recall that unqualified-ids
are contained in qualified-ids:

qualified-id:
::optnested-name-specifiertemplateoptunqualified-id

Is the wording intending to say "used as an unqualified-id
that is not part of a qualified-id?" Or something else?

Here, it is not clear whether the friend declaration denotes
A B::C() or A::B C(), yet the standard does not resolve this
ambiguity.

The ambiguity arises since both the simple-type-specifier
(7.1.5.2
dcl.type.simple
paragra 1) and an
init-declararator
(8
dcl.decl
paragraph 1)
contain an optional :: and an optional
nested-name-specifier
(5.1
expr.prim
paragraph 1).
Therefore, two different ways to analyse this code are
possible:

Since it is a friend declaration, the init-declarator may be
qualified, and start with a global scope.

Suggested Resolution: In the definition of
nested-name-specifier, add a
sentence saying that a :: token immediately following a
nested-name-specifier is always considered as part of the
nested-name-specifier. Under this interpretation, the example is
ill-formed, and should be corrected as either

It was pointed out that the proposed resolution does not deal with
cases like X::Y where X is a type but not a class type.
The working group reaffirmed its decision that the disambiguation should
be syntactic only, i.e., it should depend only on whether or not the name is a
type.

Jason Merrill
:

At the Seattle meeting, I suggested that a solution might be to change the
class-or-namespace-name in the nested-name-specifier
rule to just be
"identifier"; there was some resistance to this idea. FWIW, I've tried
this in g++. I had to revise the idea so that only the second and
subsequent names were open to being any identifier, but that seems to work
just fine.

Or some equivalent but right-associative formulation, if people feel that's
important, but it seems irrelevant to me.

Clark Nelson
:

Personally, I prefer the left-associative rule. I think it makes it easier
to understand. I was thinking about this production a lot at the meeting,
considering also some issues related to 301.
My formulation was getting kind
of ugly, but with a left-associative rule, it gets a lot nicer.

Your proposal isn't complete, however, as it doesn't allow template
arguments without an explicit template keyword. You probably want to add an
alternative for:

nested-name-specifier type-or-namespace-name::

There is admittedly overlap between this alternative and

nested-name-specifier identifier::

but I think they're both necessary.

Notes from the 4/02 meeting:

The changes look good. Clark Nelson will merge the two proposals
to produce a single proposed resolution.

Proposed resolution (April 2003):

nested-name-specifier is currently defined in
5.1
expr.prim paragraph 7 as:

Issue 215
is addressed by using type-name instead of class-name in the first alternative.
Issue 125 (this issue)
is addressed by using identifier instead of anything more specific in the third alternative.
Using left association instead of right association helps eliminate the need for class-or-namespace-name (or type-or-namespace-name, as suggested for issue 215).

It should be noted that this formulation also rules out the possibility of A::template B::, i.e. using the template keyword without any template arguments.
I think this is according to the purpose of the template keyword, and that the former rule allowed such a construct only because of the difficulty of formulation of a right-associative rule that would disallow it.
But I wanted to be sure to point out this implication.

113.
Visibility of called function

If no declaration of the called function is visible from the scope of
the call the program is ill-formed.

I think nothing there or in the previous paragraph indicates that
this does not apply to calls through pointer or virtual calls.

Mike Miller:
"The called function" is unfortunate phraseology; it makes it
sound as if it's referring to the function actually called, as
opposed to the identifier in the postfix expression. It's
wrong with respect to Koenig lookup, too (the declaration need
not be visible if it can be found in a class or namespace
associated with one or more of the arguments).

In fact, this paragraph should be a note. There's a general
rule that says you have to find an unambiguous declaration of
any name that is used
(3.4
basic.lookup
paragraph 1);
the only reason this paragraph
is here is to contrast with C's implicit declaration of called
functions.

If no declaration of the called function is visible from the
scope of the call the program is ill-formed.

to:

[Note: if a function or member function name is used,
and name lookup (3.4
basic.lookup) does not find a
declaration of that name,
the program is ill-formed. No function is implicitly declared by such
a call. ]

When there is no parameter for a given argument, the argument is
passed in such a way that the receiving function can obtain the
value of the argument by invoking va_arg
(18.7
lib.support.runtime). The lvalue-to-rvalue
(4.1
conv.lval), array-to-pointer
(4.2
conv.array), and function-to-pointer
(4.3
conv.func) standard conversions are performed
on the argument expression. After these conversions, if the
argument does not have arithmetic, enumeration, pointer,
pointer to member, or class type, the program is ill-formed.
If the argument has a non-POD class type (clause
9
class), the behavior is undefined.

Paper J16/04-0167=WG21 N1727 suggests that passing a non-POD
object to ellipsis be ill-formed. In discussions at the
Lillehammer meeting, however, the CWG felt that the newly-approved
category of conditionally-supported behavior would be more
appropriate.

...After these conversions, if the argument does not have arithmetic,
enumeration, pointer, pointer to member, or class type, the program is
ill-formed. If the argument has a non-POD class type (clause 9),
the behavior is undefined.Passing an argument of non-POD class
type (clause 9) with no corresponding parameter is
conditionally-supported, with implementation-defined semantics.

466.
cv-qualifiers on pseudo-destructor type

The type designated by the pseudo-destructor-name shall be the same
as the object type.

and also:

The cv-unqualified versions of the object type and of the type
designated by the pseudo-destructor-name shall be the same type.

Which is it? "The same" or "the same up to cv-qualifiers"? The
second sentence is more generous than the first. Most compilers seem
to implement the less restrictive form, so I guess that's what I think
we should do.

The left-hand side of the dot operator shall be of scalar type. The
left-hand side of the arrow operator shall be of pointer to scalar
type. This scalar type is the object type. The type designated by the
pseudo-destructor-name shall be the same as the object
type.The cv-unqualified versions of
the object type and of the type designated by the
pseudo-destructor-name shall be the same type. Furthermore,
the two type-names in a pseudo-destructor-name of the
form

::opt nested-name-specifieropt type-name ::~ type-name

shall designate the same scalar type. The cv-unqualified versions of
the object type and of the type designated by the
pseudo-destructor-name shall be the same type.

492.
typeid constness inconsistent with example

There is an inconsistency between the normative text in
section 5.2.8
expr.typeid and
the example that follows.

Here is the relevant passage (starting with paragraph 4):

When typeid is applied to a type-id, the result
refers to a std::type_info object representing the type of the
type-id. If the type of the type-id is a reference
type, the result of the typeid expression refers to a
std::type_info object representing the referenced type.

The top-level cv-qualifiers of the lvalue expression or the
type-id that is the operand of typeid are always
ignored.

and the example:

typeid(D) == typeid(const D&); // yields true

The second paragraph above says the “type-id that is
the operand”. This would be const D&. In
this case, the const is not at the top-level (i.e.,
applied to the operand itself).

By a strict reading, the above should yield false.

My proposal is that the strict reading of the normative test is correct.
The example is wrong. Different compilers here give different
answers.

Proposed resolution (April, 2005):

Change the second sentence of 5.2.8
expr.typeid
paragraph 4 as follows:

If the type of the type-id is a reference to a possibly
cv-qualified type, the result of the typeid expression
refers to a std::type_info object representing the
cv-unqualified referenced type.

54.
Static_cast from private base to derived class

Is it okay to use a static_cast to cast from a private base
class to a derived class? That depends on what the words "valid standard
conversion" in paragraph 8 mean — do they mean the conversion exists,
or that it would not get an error if it were done? I think the former
was intended — and therefore a static_cast from a private base
to a derived class would be allowed.

Rationale (04/99): A static_cast from a private
base to a derived class is not allowed outside a member from the
derived class, because 4.10
conv.ptr
paragraph 3 implies that the conversion is not valid. (Classic style
casts work.)

Reopened September 2003:

Steve Adamczyk:
It makes some sense to disallow the inverse conversion that is
pointer-to-member of derived to pointer-to-member of private base.
There's less justification for the pointer-to-private-base to
pointer-to-derived case. EDG, g++ 3.2, and MSVC++ 7.1 allow
the pointer case and disallow the pointer-to-member case.
Sun disallows the pointer case as well.

There's a tricky case with old-style casts: because the
static_cast interpretation is tried first, you want a case
like the above to be considered a static_cast, but then issue
an error, not be rejected as not a static cast; if you did the
latter, you would then try the cast as a reinterpret_cast.

Ambiguity and casting to a virtual base should likewise be
errors after the static_cast interpretation is selected.

Notes from the October 2003 meeting:

There was lots of sentiment for making things symmetrical:
the pointer case should be the same as the pointer-to-member case.
g++ 3.3 now issues errors on both cases.

We decided an error should be issued on both cases.
The access part of the check should be done later; by some
definition of the word the static_cast is valid, and then
later an access error is issued.
This is similar to the way standard conversions work.

The inverse of any standard conversion sequence (clause
4
conv), other than the
lvalue-to-rvalue (4.1
conv.lval),
array-to-pointer (4.2
conv.array),
function-to-pointer (4.3
conv.func), and
boolean (4.12
conv.bool) conversions, can be performed
explicitly using static_cast.
The lvalue-to-rvalue (4.1
conv.lval),
array-to-pointer (4.2
conv.array), and
function-to-pointer (4.3
conv.func)
conversions are applied to the operand.
Such a
static_cast
is subject to the restriction that the explicit conversion does not
cast away constness (5.2.11
expr.const.cast), and the following
additional rules for specific cases:

with two paragraphs:

The inverse of any standard conversion sequence (clause
4
conv), other than the
lvalue-to-rvalue (4.1
conv.lval),
array-to-pointer (4.2
conv.array),
function-to-pointer (4.3
conv.func), and
boolean (4.12
conv.bool) conversions, can be performed
explicitly using static_cast.
A program is ill-formed if it
uses static_cast to perform the inverse of an ill-formed
standard conversion sequence.[Example:

The lvalue-to-rvalue (4.1
conv.lval),
array-to-pointer (4.2
conv.array), and
function-to-pointer (4.3
conv.func)
conversions are applied to the operand.
Such a
static_cast
is subject to the restriction that the explicit conversion does not
cast away constness (5.2.11
expr.const.cast), and the following
additional rules for specific cases:

In addition, modify the second sentence of
5.4
expr.cast/5. The first two
sentences of 5.4
expr.cast/5 presently read:

The conversions performed by

a const_cast (5.2.11),

a static_cast (5.2.9),

a static_cast followed by a const_cast,

a reinterpret_cast (5.2.10), or

a reinterpret_cast followed by a const_cast,

can be performed using the cast notation of explicit type conversion.
The same semantic restrictions and behaviors apply.

Change the second sentence to read:

The same semantic restrictions and behaviors apply, with the exception
that in performing a static_cast in the following situations
the conversion is valid even if the base class is inaccessible:

a pointer to an object of derived class type or an lvalue of
derived class type may be explicitly converted to a pointer or
reference to an unambiguous base class type, respectively;

a pointer to member of derived class type may be explicitly
converted to a pointer to member of an unambiguous non-virtual base
class type;

a pointer to an object of an unambiguous non-virtual
base class type, an lvalue of an unambiguous non-virtual base
class type, or a pointer to member of an unambiguous
non-virtual base class type may be explicitly converted to a pointer,
a reference, or a pointer to member of a derived class type,
respectively.

In addition to
those conversions,
the following
static_cast
and
reinterpret_cast
operations
(optionally followed by a
const_cast
operation)
may be performed using the cast notation of explicit type conversion,
even if the base class type is not accessible:

a pointer to an object of derived class type or an lvalue of derived class
type may be explicitly converted
to a pointer or reference to an unambiguous base class type, respectively;

a pointer to member of derived class type may be explicitly converted
to a pointer to member of an unambiguous non-virtual base class type;

a pointer to an object of non-virtual base class type, an lvalue
of non-virtual base class type, or a pointer to member of non-virtual
base class type may be explicitly converted to a pointer, a reference,
or a pointer to member of a derived class type, respectively.

Presumably the first alternative is better -- it's the "simpler"
conversion. The standard does not seem to make that clear.

Steve Adamczyk:
I take the "Otherwise" at the beginning of
5.2.9
expr.static.cast/3 as meaning that
the paragraph 2 interpretation is used if available, which means in
your example above interpretation 2 would be used. However, that's
not what EDG's compiler does, and I agree that it's not the "simpler"
conversion.

Proposed Resolution (October 2003):

Move paragraph 5.2.9/5:

An lvalue of type
``cv1B'',
where
B
is a class type, can be cast to type ``reference to
cv2D'',
where
D
is a class derived (clause
10
class.derived) from
B,
if a valid standard conversion from ``pointer to
D''
to ``pointer to
B''
exists (4.10
conv.ptr),
cv2
is the same cv-qualification as, or greater cv-qualification than,
cv1,
and
B
is not a virtual base class of
D.
The result is an lvalue of type
``cv2D.''
If the lvalue of type
``cv1B''
is actually a sub-object of an object of type
D,
the lvalue refers to the enclosing object of type
D.
Otherwise, the result of the cast is undefined.
[Example:

Insert Otherwise, before the text of paragraph
5.2.9
expr.static.cast/2
(which will become 5.2.9
expr.static.cast/3
after the above insertion), so that it
reads:

Otherwise, an expression
e
can be explicitly converted to a type
T
using a
static_cast
of the form
static_cast<T>(e)
if the declaration
"T t(e);"
is well-formed, for some invented temporary variable
t
(8.5
dcl.init).
The effect of such an explicit conversion is the same as performing the
declaration and initialization and then using the temporary variable
as the result of the conversion.
The result is an lvalue if
T
is a reference type (8.3.2
dcl.ref), and an rvalue otherwise.
The expression
e
is used as an lvalue if and only if the initialization uses it as an lvalue.

439.
Guarantees on casting pointer back to cv-qualified version of original type

there should be no call to "abort". The last sentence of Paragraph
5.2.9
expr.static.cast paragraph 10 should be changed to read:

A value of type pointer to object converted to "pointer to
cvvoid" and back to the original pointer type
(or a variant of the original pointer type that differs only in the
cv-qualifiers applied to the object type) will have its
original value.
[Example:

Rationale: The wording "possibly with different
cv-qualification" was chosen over the suggested wording to
allow for changes in cv-qualification at different levels in a
multi-level pointer, rather than only at the object type level.

195.
Converting between function and object pointers

It is currently not permitted to cast directly between a pointer to
function type and a pointer to object type. This conversion is not
listed in 5.2.9
expr.static.cast
and
5.2.10
expr.reinterpret.cast
and thus
requires a diagnostic to be issued. However, if a sufficiently long
integral type exists (as is the case in many implementations), it is
permitted to cast between pointer to function types and pointer to
object types using that integral type as an intermediary.

In C the cast results in undefined behavior and thus does not
require a diagnostic, and Unix C compilers generally do not issue
one. This fact is used in the definition of the standard Unix
function dlsym, which is declared to return void*
but in fact may return either a pointer to a function or a pointer to
an object. The fact that C++ compilers are required to issue a
diagnostic is viewed as a "competitive disadvantage" for the language.

Suggested resolution: Add wording to
5.2.10
expr.reinterpret.cast
allowing
conversions between pointer to function and pointer to object types,
if the implementation has an integral data type that can be used as an
intermediary.

Several points were raised in opposition to this suggestion:

Early C++ supported this conversion and it was deliberately
removed during the standardization process.

The existence of an appropriate integral type is irrelevant to
whether the conversion is "safe." The condition should be on whether
information is lost in the conversion or not.

There are numerous ways to address the problem at an
implementation level rather than changing the language. For example,
the compiler could recognize the specific case of dlsym and
omit the diagnostic, or the C++ binding to dlsym could be
changed (using templates, for instance) to circumvent the violation.

The conversion is, in fact, not supported by C; the dlsym
function is simply relying on non-mandated characteristics of C
implementations, and we would be going beyond the requirements of C
compatibility in requiring (some) implementations to support the
conversion.

This issue is in fact not a defect (omitted or self-contradictory
requirements) in the current Standard; the proposed change would
actually be an extension and should not be considered until the full
review of the IS.

dlsym appears not to be used very widely, and the
declaration in the header file is not problematic, only calls to it.
Since C code generally requires some porting to be valid C++ anyway,
this should be considered one of those items that requires porting.

Martin O'Riordan suggested an alternative approach:

Do not allow the conversions to be performed via old-style casts.

Allow reinterpret_cast to perform the conversions with
the C-style semantics (undefined behavior if not supported).

Allow dynamic_cast to perform the conversions with
well-defined behavior: the result would be a null pointer if the
implementation could not support the requested conversion.

The advantage of this approach is that it would permit writing
portable, well-defined programs involving such conversions. However,
it breaks the current degree of compatibility between old and new
casts, and it adds functionality to dynamic_cast which is not
obviously related to its current meaning.

Notes from 04/00 meeting:

Andrew Koenig suggested yet another approach: specify that "no
diagnostic is required" if the implementation supports the conversion.

Later note:

It was observed that conversion between function and data pointers
is listed as a "common extension" in C99.

Notes on the 10/01 meeting:

It was decided that we want the conversion defined in such a way that
it always exists but is always undefined (as opposed to existing only
when the size relationship is appropriate, and being implementation-defined
in that case). This would allow an implementation to issue an error
at compile time if the conversion does not make sense.

Bill Gibbons notes that the definitions of the other similar casts are
inconsistent in this regard. Perhaps they should be updated as well.

A pointer to a
function can be explicitly converted to a pointer to a function of a different
type. The effect of calling a function through a pointer to a function
type (8.3.5
dcl.fct)
that is not the same as the type used in the definition of the function is
undefined. Except that converting an rvalue of type ``pointer to T1''
to the type ``pointer to T2'' (where T1 and T2 are
function types) and back to its original type yields the original pointer
value, the result of such a pointer conversion is unspecified. [Note: see
also 4.10
conv.ptr for more details of pointer conversions. ]
It is implementation defined whether a conversion from pointer to object to
pointer to function and/or a conversion from pointer to function to pointer to
object exist.

and in paragraph 10:

An lvalue expression of type T1 can be cast to the type
``reference to T2''
if T1 and T2 are object types and an expression of
type ``pointer to T1'' can be explicitly converted to the type
``pointer to T2'' using a reinterpret_cast. That is, a reference
cast reinterpret_cast< T& >(x) has the same effect
as the conversion *reinterpret_cast< T* >(&x) with
the built-in & and * operators.
The result is an lvalue that refers to the same object as the source lvalue,
but with a different type. No temporary is created, no copy is made, and
constructors (12.1
class.ctor) or conversion functions
(12.3
class.conv) are not called.

Drafting Note:

If either conversion exists, the implementation already has to define the
behavior (paragraph 3).

Notes from April 2003 meeting:

The new consensus is that if the implementation allows
this cast, pointer-to-function converted to pointer-to-object converted
back to the original pointer-to-function should work; anything else
is undefined behavior. If the implementation does not allow the cast,
it should be ill-formed.

Tom Plum is investigating a new concept, that of a
"conditionally-defined" feature, which may be applicable here.

Proposed Resolution (October, 2004):

(See paper J16/04-0067 = WG21 N1627 for background material and
rationale for this resolution. The resolution proposed here differs
only editorially from the one in the paper.)

Insert the following as 1.3.2 and renumber all following
definitions accordingly:

1.3.2 conditionally-supported behavior

behavior evoked by a program construct that is not a mandatory
requirement of this International Standard. If a given implementation
supports the construct, the behavior shall be as described herein;
otherwise, the implementation shall document that the construct is not
supported and shall treat a program containing an occurrence of the
construct as ill-formed (1.3.4
defns.ill.formed).

If a program contains a violation of any diagnosable
rule, or an occurrence of a construct described herein as
“conditionally-supported” or as resulting in
“conditionally-supported behavior” when the implementation
does not in fact support that construct, a conforming
implementation shall issue at least one diagnostic message, except
that

Converting a pointer to a function to a pointer to an object type or
vice versa evokes conditionally-supported behavior. In any such
conversion supported by an implementation, converting from an rvalue
of one type to the other and back (possibly with different
cv-qualification) shall yield the original pointer value; mappings
between pointers to functions and pointers to objects are otherwise
implementation-defined.

The string-literal indicates the required language linkage. The meaning of the
string-literal is implementation-defined. A linkage-specification with a string that
is unknown to the implementation is ill-formed.This International Standard
specifies the semantics of C and C++ language linkage. Other values of the
string-literal evoke conditionally-supported behavior, with
implementation-defined semantics. [Note: Therefore, a
linkage-specification with a string-literal that is
unknown to the implementation requires a diagnostic.When the
string-literal in a linkage-specification names a programming
language, the spelling of the programming language's name is
implementation-defined. [Note:It is recommended that the
spelling be taken from the document defining that language, for
example Ada (not ADA) and Fortran or
FORTRAN (depending on the vintage). The semantics of a
language linkage other than C++ or C are implementation-defined. ]

A template, a template explicit specialization (14.7.3
temp.expl.spec), or a class template partial specialization shall
not have C linkage. If the linkage of one of these is something other
than C or C++, the behavior is implementation-definedresult
is conditionally-supported behavior, with implementation-defined
semantics.

Converting an integral constant expression (5.19) with value zero
always yields a null pointer (4.10), but converting other
expressions that happen to have value zero need not yield a null
pointer.

Where is this supported in normative text? It seems that either the
footnote or paragraph 8 doesn't reflect the intent.

SUGGESTED RESOLUTION: I think it would be better to drop the footnote
#64 (and thus the special case for ICEs), for two reasons:

a) it's not normative anyway; so I doubt anyone is relying on the
guarantee it hints at, unless that guarantee is given elsewhere in a
normative part

b) users expect reinterpret_casts to be almost always implementation
dependent, so this special case is a surprise. After all, if one wants
a null pointer there's static_cast. And if one wants reinterpret_cast
semantics the special case requires doing some explicit cheat, such as
using a non-const variable as intermediary:

It seems that not only that's providing a duplicate functionality, but
also at the cost to hide what seems the more natural one.

Notes from October 2004 meeting:

This footnote was added in 1996, after the invention of
reinterpret_cast, so the presumption must be that it was
intentional. At this time, however, the CWG feels that there is
no reason to require that reinterpret_cast<T*>(0)
produce a null pointer value as its result.

The null pointer value (4.10
conv.ptr) is converted to
the null pointer value of the destination type. [Note:
A null pointer constant, which has integral type, is not necessarily
converted to a null pointer value. —end note]

If the second and third operands are lvalues and have
the same type, the result is of that type and is an lvalue
and it is a bit-field if the second or the third
operand is a bit-field, or if both are bit-fields.

In 5.17
expr.ass paragraph 1 (assignment operators)
add the indicated text (the original text is
as updated by issue 221, which is DR but not in TC1):

The assignment operator (=) and the compound assignment
operators all group right-to-left. All require a modifiable lvalue as
their left operand and return an lvalue with the type and value of
the left operand after the assignment has taken place. The result
in all cases is a bit-field if the left operand is a bit-field.

Note that issue 222 adds (non-conflicting) text at the end of
this same paragraph (5.17
expr.ass paragraph 1).

The type and value of the result are the type and value of the right
operand; the result is an lvalue if its right operand is.

to

The type and value of the result are the type and value of the right
operand; the result is an lvalue if the right operand is an lvalue,
and is a bit-field if the right operand is an lvalue and a bit-field.

299.
Conversion on array bound expression in new

In 5.3.4
expr.new, the standard says that the
expression in an array-new has to
have integral type. There's already a DR (issue 74)
that says it should also be allowed to have enumeration type. But, it should
probably also say that it can have a class type with a single
conversion to integral type; in other words the same thing as in
6.4.2
stmt.switch paragraph 2.

Suggested resolution:

In 5.3.4
expr.new paragraph 6, replace
"integral or enumeration type (3.9.1
basic.fundamental)" with
"integral or enumeration type (3.9.1
basic.fundamental), or a class
type for which a single conversion function to
integral or enumeration type exists".

Every constant-expression in a
direct-new-declarator shall be an integral constant
expression (5.19
expr.const) and evaluate to a
strictly positive value. The expression in a
direct-new-declarator shall havebe of
integral type, or enumeration type (3.9.1), or a
class type for which a single conversion function to integral or
enumeration type exists (12.3
class.conv). If the
expression is of class type, the expression is converted by
calling the conversion function, and the result of the conversion
is used in place of the original expression. The value of the
expression shall bewith a non-negative
value. [Example: ...

Every constant-expression in a
direct-new-declarator shall be an integral constant
expression (5.19
expr.const) and evaluate to a
strictly positive value. The expression in a
direct-new-declarator shall have integral or enumeration
type (3.9.1
basic.fundamental) with a non-negative
valuebe of integral type, enumeration type, or a class type for
which a single conversion function to integral or enumeration type
exists (12.3
class.conv). If the expression is of class
type, the expression is converted by calling that conversion function,
and the result of the conversion is used in place of the original
expression. If the value of the expression is negative, the behavior
is undefined. [Example: ...

429.
Matching deallocation function chosen based on syntax or signature?

The fundamental issue here is whether the deletion-on-throw
is driven by the syntax of the new (placement or non-placement)
or by signature matching. If the former, the operator delete
would be called with the second argument equal to the size of
the class. If the latter, it would be called with the second
argument 0.

Core issue 127 (in TC1) dealt with
this topic. It removed some wording
in 15.2
except.ctor paragraph 2 that implied a
syntax-based interpretation, leaving wording
in 5.3.4
expr.new paragraph 19 that is
signature-based. But there is no accompanying
rationale to confirm an explicit choice of the signature-based
approach.

EDG and g++ get 0 for the second argument, matching the presumed
core issue 127 resolution. But maybe this should be revisited.

Notes from October 2003 meeting:

There was widespread agreement that the compiler shouldn't
just silently call the delete with either of the possible values.
In the end, we decided it's smarter to issue an error on this
case and force the programmer to say what he means.

Mike Miller's analysis of the status quo:
3.7.3.2
basic.stc.dynamic.deallocation paragraph 2
says that "operator delete(void*, std::size_t)" is a
"usual (non-placement) deallocation function" if the class does
not declare "operator delete(void*)."
3.7.3.1
basic.stc.dynamic.allocation does not use the
same terminology for allocation functions, but the most
reasonable way to understand the uses of the term "placement
allocation function" in the Standard is as an allocation function
that has more than one parameter and thus can (but need not) be
called using the "new-placement" syntax described in
5.3.4
expr.new. (In
considering issue 127,
the core group discussed and endorsed the
position that, "If a placement allocation function has default
arguments for all its parameters except the first, it can be
called using non-placement syntax.")

5.3.4
expr.new paragraph 19
says that any non-placement deallocation function
matches a non-placement allocation function, and that a placement
deallocation function matches a placement allocation function
with the same parameter types after the first -- i.e., a
non-placement deallocation function cannot match a placement
allocation function. This makes sense, because non-placement
("usual") deallocation functions expect to free memory obtained
from the system heap, which might not be the case for storage
resulting from calling a placement allocation function.

According to this analysis, the example shows a placement
allocation function and a non-placement deallocation function, so
the deallocation function should not be invoked at all, and the
memory will just leak.

If the lookup finds the two-parameter form of a usual deallocation function
(3.7.3.2
basic.stc.dynamic.deallocation), and that function, considered as
a placement deallocation function, would have been selected as a match
for the allocation function, the program is ill-formed.
[Example:

353.
Is deallocation routine called if destructor throws exception in delete?

In a couple of comp.std.c++ threads, people have asked
whether the Standard guarantees that the deallocation function
will be called in a delete-expression if the destructor throws
an exception. Most/all people have expressed the opinion that
the deallocation function must be called in this case, although
no one has been able to cite wording in the Standard supporting
that view.

The delete-expression will call a deallocation function
(3.7.3.2
basic.stc.dynamic.deallocation)
[Note: The deallocation function is called regardless of
whether the destructor for the object or some element of the array
throws an exception. ]

442.
Incorrect use of null pointer constant in description of delete operator

After some discussion in comp.lang.c++.moderated we came to the conclusion
that there seems to be a defect in
5.3.5
expr.delete/4, which says:

The cast-expression in a delete-expression shall be evaluated exactly once.
If the delete-expression calls the implementation deallocation function
(3.7.3.2), and if the operand of the delete expression is not the null
pointer constant, the deallocation function will deallocate the storage
referenced by the pointer thus rendering the pointer invalid. [Note: the
value of a pointer that refers to deallocated storage is indeterminate. ]

In the second sentence, the term "null pointer constant" should be changed
to "null pointer". In its present form, the passage claims that the
deallocation function will deallocate the storage refered to by a null
pointer that did not come from a null pointer constant in the delete
expression. Besides, how can the null pointer constant be the operand of a
delete expression, as "delete 0" is an error because delete requires a
pointer type or a class type having a single conversion function to a
pointer type?

If the delete-expression calls the implementation deallocation
function (3.7.3.2
basic.stc.dynamic.deallocation), and if
the value of the operand of
the delete expression is not thea null
pointer constant, the deallocation function will deallocate the
storage referenced by the pointer thus rendering the pointer invalid.

Notes from October 2004 meeting:

This wording is superseded by, and this issue will be resolved
by, the resolution of issue 348.

520.
Old-style casts between incomplete class types

The operand of a cast using the cast notation can be an rvalue of type
“pointer to incomplete class type”. The destination type
of a cast using the cast notation can be “pointer to incomplete
class type”. In such cases, even if there is a inheritance
relationship between the source and destination classes, whether the
static_cast or reinterpret_cast interpretation is
used is unspecified.

It's not entirely clear how a compiler can
choose static_cast as 5.4
expr.cast paragraph 6
seems to allow. I believe the intent of 5.4
expr.cast
paragraph 6 is to force the use of reinterpret_cast when
either are incomplete class types and static_cast iff the
compiler knows both types and there is a non-ambiguous
hierarchy-traversal between that cast (or maybe not, core issue 242 talks about this). I cannot see any
other interpretation because it isn't intuitive, every compiler I've
tried agrees with me, and neither standard pointer conversions
(4.10
conv.ptr paragraph 3) nor static_cast
(5.2.9
expr.static.cast paragraph 5) talk about incomplete
class types. If the committee agrees with me, I would like to see
4.10
conv.ptr paragraph 3 and 5.2.9
expr.static.cast paragraph 5 explicitly disallow incomplete class
types and the wording of 5.4
expr.cast paragraph 6
changed to not allow any other interpretation.

The operand of a cast using the cast notation can be an rvalue of type
“pointer to incomplete class type.” The destination type
of a cast using the cast notation can be “pointer to incomplete
class type.” In such cases, even if there is a inheritance
relationship between the source and destination classes, whether
the static_cast or reinterpret_cast interpretation
is used is unspecified.If both the operand and destination types
are class types and one or both are incomplete, it is unspecified
whether the static_cast or the reinterpret_cast
interpretation is used, even if there is an inheritance relationship
between the two classes. [Note: For example, if the classes
were defined later in the translation unit, a multi-pass compiler
would be permitted to interpret a cast between pointers to the classes
as if the class types were complete at that point. —end
note]

446.
Does an lvalue-to-rvalue conversion on the "?" operator produce a temporary?

The problem occurs when the value of the operator is determined to
be an rvalue, the selected argument is an lvalue, the type is a class
type and a non-const member is invoked on the modifiable rvalue result.

The types of the second and third operands are the same and
one is an rvalue. Nothing changes until p6 where an lvalue to
rvalue conversion is performed on the third operand.
12.2
class.temporary states that an lvalue to rvalue
conversion produces a temporary and there is nothing to remove
it. It seems clear that the assertion must pass, yet most
implementations fail.

There seems to be a defect in p3 b2 b1. First, the conditions to get
here and pass the test.

If E1 and E2 have class type, and the underlying class types are the
same or one is a base class of the other: E1 can be converted to
match E2 if the class of T2 is the same type as, or a base class of,
the class of T1, and the cv-qualification of T2 is the same
cv-qualification as, or a greater cv-qualification than, the
cv-qualification of T1.

If both E1 and E2 are lvalues, passing the conditions here also passes
the conditions for p3 b1. Thus, at least one is an rvalue. The case of
two rvalues is not interesting and the action is covered by the case
when E1 is an rvalue.

(0 ? D(13) : b1).inc();
assert(b1.v == 42);

E1 is changed to an rvalue of type T2 that still refers to the
original source class object (or the appropriate subobject thereof).
[Note: that is, no copy is made. ]

Having changed the rvalue to base type, we are back to the above case
where an lvalue to rvalue conversion is required on the third operand
at p6. Again, most implementations fail.

The remaining case, E1 an lvalue and E2 an rvalue, is the defect.

D d1(42);
(0 ? B(13) : d1).inc();
assert(d1.v == 42);

The above quote states that an lvalue of type T1 is changed to an rvalue
of type T2 without creating a temporary. This is in contradiction to
everything else in the standard about lvalue to rvalue conversions.
Most implementations pass in spite of the defect.

The usual accessible and unambiguous is missing from the base class.

There seems to be two possible solutions. Following other temporary
creations would produce a temporary rvalue of type T1 and change it
to an rvalue of type T2. Keeping the no copy aspect of this bullet
intact would change the lvalue of type T1 to an lvalue of type T2.
In this case the lvalue to rvalue conversion would happen in p6 as
usual.

Suggested wording for p3 b2 b1

The base part:

If E1 and E2 have class type, and the underlying class types are the
same or one is a base class of the other: E1 can be converted to match
E2 if the class of T2 is the same type as, or an accessible and
unambiguous base class of, the class of T1, and the cv-qualification
of T2 is the same cv-qualification as, or a greater cv-qualification
than, the cv-qualification of T1. If the conversion is applied:

The same type temporary version:

If E1 is an lvalue, an lvalue to rvalue conversion is applied. The
resulting or original rvalue is changed to an rvalue of type T2 that
refers to the same class object (or the appropriate subobject
thereof). [Note: that is, no copy is made in changing the type of
the rvalue. ]

The never copy version:

The lvalue(rvalue) E1 is changed to an lvalue(rvalue) of type T2
that refers to the original class object (or the appropriate
subobject thereof). [Note: that is, no copy is made. ]

The test case was posted to clc++m and results for implementations
were reported.

I see no defect with regards to lvalue to rvalue conversions; however,
there seems to be disagreement about what it means by implementers.
It may not be surprising because 5.16 and passing a POD struct to an
ellipsis are the only places where an lvalue to rvalue conversion
applies to a class type. Most lvalue to rvalue conversions are on
basic types as operands of builtin operators.

Notes from the March 2004 meeting:

We decided all "?" operators that return a class rvalue should
copy the second or third operand to a temporary. See
issue 86.

if E1 and E2 have class type, and the
underlying class types are the same or one is a base class of the
other: E1 can be converted to match E2 if the
class of T2 is the same type as, or a base class of, the
class of T1, and the cv-qualification of T2 is
the same cv-qualification as, or a greater cv-qualification than,
the cv-qualification of T1. If the conversion is
applied, E1 is changed to an rvalue of type T2that still refers to the original source class object (or the
appropriate subobject thereof). [Note: that is, no copy is
made. —end note]by copy-initializing a
temporary of type T2 from E1 and using that
temporary as the converted operand.

The second and third operands have the same type; the result is
of that type. If the operands have class type, the result is an
rvalue temporary of the result type, which is copy-initialized
from either the second operand or the third operand depending on
the value of the first operand.

The value contained in the object indicated by the lvalue is
the rvalue result. When an lvalue-to-rvalue conversion occurs
within the operand of sizeof (5.3.3
expr.sizeof) the value contained in the referenced object is
not accessed, since that operator does not evaluate its operand.
Otherwise, if the lvalue has a class type, the conversion
copy-initializes a temporary of type T from the lvalue
and the result of the conversion is an rvalue for the temporary.
Otherwise, the value contained in the object indicated by the
lvalue is the rvalue result.

366.
String literal allowed in integral constant expression?

is well-formed, since it is an integral constant expression.
Since that may not be obvious, here is why:

5.19
expr.const paragraph 1 says that an integral constant
expression may involve literals (2.13
lex.literal);
"Hello, world" is a literal. It restricts
operators to not use certain type conversions; this expression does
not use type conversions. It further disallows functions, class
objects, pointers, ... - this expression is none of those, since it
is an array.

However, 16.1
cpp.cond paragraph 6 does not explain what
to do with this if-group, since
the expression evaluates neither to false(zero) nor true(non-zero).

Proposed resolution (October 2002):

Change the beginning of the second sentence of
5.19
expr.const paragraph 1 which currently reads

An integral constant-expression can involve only literals
(2.13
lex.literal), ...

According to 5.19
expr.const, this appears to be an
integral constant expression:
it is a conditional expression, involves only literals, and no
assignment, increment, decrement, function-call, or comma operators.
However, if this is well-formed, the standard gives no meaning to
this declaration, since the array bound (8.3.4
dcl.array
paragraph 1) cannot be computed.

I believe the defect is that throw expressions should also be banned
from constant expressions.

Notes from October 2002 meeting:

We should also check on new and delete.

Notes from the April, 2005 meeting:

Although it could be argued that all three of these operators
potentially involve function calls — throw
to std::terminate, new and delete to the
corresponding allocation and deallocation functions — and thus
would already be excluded from constant expressions, this reasoning
was considered to be too subtle to allow closing the issue with no
change. A modification that explicitly clarifies the status of these
operators will be drafted.

Bjarne Stroustrup:
... and [the ARM] disallows declaring a function inline after it
has been called.

This may still be a good resolution.

Steve Clamage:
But the situation in the ARM is the reverse: Declare a function
inline, and define it later (with no intervening call). That's
a long-standing rule in C++, and allows you to write member
function definitions outside the class.

In my example, the compiler could reasonably process the entire
function as out-of-line, and not discover the inline declaration
until it was too late to save the information necessary for inline
generation. The equivalent of another compiler pass would be needed
to handle this situation.

Bjarne Stroustrup:
I see, and I think your argument it conclusive.

Steve Clamage:
I'd like to open a core issue on this point, and I recommend wording
along the lines of: "A function defined without an inline specifier
shall not be followed by a declaration having an inline specifier."

I'd still like to allow the common idiom

class T {
int f();
};
inline int T::f() { ... }

Martin Sebor:
Since the inline keyword is just a hint to the compiler, I don't see
any harm in allowing the construct. Your hypothetical compiler can
simply ignore the inline on the second declaration. On the other hand,
I feel that adding another special rule will unnecessarily complicate
the language.

Steve Clamage:
The inline specifier is more than a hint. You can have multiple
definitions of inline functions, but only one definition of a
function not declared inline. In particular, suppose the above
example were in a header file, and included multiple times in
a program.

An inline function shall be defined in every translation unit in
which it is used and shall have exactly the same definition in
every case (3.2
basic.def.odr). [Note: a call to
the inline function may be encountered before its definition
appears in the translation unit. —end note] If the
definition of a function appears in a translation unit before its
first declaration as inline, the program is ill-formed. If a
function with external linkage is declared inline in one
translation unit...

396.
Misleading note regarding use of auto for disambiguation

BTW, I noticed that the following note in 7.1.1
dcl.stc
paragraph 2 doesn't seem to have
made it onto the issues list or into the TR:

[Note: hence, the auto specifier is almost always redundant and not
often used; one use of auto is to distinguish a declaration-statement
from an expression-statement (stmt.ambig) explicitly. --- end note]

I thought that this was well known to be incorrect, because using auto
does not disambiguate this. Writing:

auto int f();

is still a declaration of a function f, just now with an error since the
function's return type may not use an auto storage class specifier. I
suppose an error is an improvement over a silent ambiguity going the
wrong way, but it's still not a solution for the user who wants to
express the other in a compilable way.

Proposed resolution: Replace that note with the following note:

[Note: hence, the auto specifier is always redundant and not often
used. --- end note]

John Spicer:
I support the proposed change, but I think the disambiguation case is not the
one that you describe. An example of the supposed disambiguation is:

cfront would take "int(i)" as a cast of ::i, so the auto would force what it
would otherwise treat as a statement to be considered a declaration
(cfront 3.0 warned that this would change in the future).

In a conforming compiler the auto is always redundant (as you say) because
anything that could be considered a valid declaration should be treated as
one.

[Note: hence, the auto specifier
is almost always redundant and not often used;
one use of auto is to distinguish
a declaration-statement from
an expression-statement (6.8
stmt.ambig)
explicitly. --- end note]

with

[Note: hence, the auto specifier
is always redundant and not often used.
One use of auto is to distinguish
a declaration-statement from an expression-statement
explicitly
rather than relying on the disambiguation rules
(6.8
stmt.ambig),
which may aid readers.
--- end note]

397.
Same address for string literals from default arguments in inline functions?

Are string literals from default arguments
used in extern inlines supposed to have the same addresses across
all translation units?

void f(const char* = "s")
inline g() {
f();
}

Must the "s" strings be the same in all copies of the
inline function?

Steve Adamczyk:
The totality of the standard's wisdom on this topic is
(7.1.2
dcl.fct.spec paragraph 4):

A string literal in an extern inline function is the same object
in different translation units.

I'd hazard a guess that a literal in a default argument expression
is not "in" the extern inline function (it doesn't appear in the
tokens of the function), and therefore it need not be
the same in different translation units.

I don't know that users would expect such strings to have
the same address, and an equally valid (and incompatible) expectation
would be that the same string literal would be used for every expansion of
a given default argument in a single translation unit.

Notes from April 2003 meeting:

The core working group feels that the address of a string literal
should be guaranteed to be the same only if it actually appears
textually within the body of the inline function. So a string in
a default argument expression
in a block extern declaration inside the body of a function would
be the same in all instances of the function. On the other
hand, a string in a default argument expression in the header of
the function (i.e., outside of the body) would not be the same.

Proposed resolution (April 2003):

Change the last sentence and add the note to the end of
7.1.2
dcl.fct.spec paragraph 4:

A string literal in the body of an extern inline
function is the same
object in different translation units.
[Note: A string literal that is encountered only in the context of a
function call (in the default argument expression of the called function),
is not “in” the extern inline function.]

Notes from October 2003 meeting:

We discussed ctor-initializer lists and decided that they are
also part of the body. We've asked Clark Nelson to work on
syntax changes to give us a syntax term for the body of a
function so we can refer to it here. See also
issue 452, which could use this term.

(October, 2005: moved to “review” status in concert with
issue 452. With that resolution, the wording
above needs no further changes.)

Proposed resolution (April, 2006):

Change the last sentence and add the note to the end of
7.1.2
dcl.fct.spec paragraph 4:

A string literal in the body of an extern inline
function is the same object in different translation units.
[Note: A string literal appearing in a default argument
expression is not considered to be “in the body”
of an inline function merely by virtue of the expression’s use in a
function call from that inline function. —end note]

477.
Can virtual appear in a friend declaration?

I couldn't find wording that makes it invalid to say
friend virtual... The closest seems to be
7.1.2
dcl.fct.spec paragraph 5, which says:

The virtual specifier shall only be used in declarations
of nonstatic class member functions that appear within a
member-specification of a class definition; see
10.3
class.virtual.

I don't think that excludes a friend declaration (which is a
valid member-specification by 9.2
class.mem).

John Spicer: I agree that virtual should not be allowed
on friend declarations. I think the wording in 7.1.2
dcl.fct.spec is intended to be the declaration of a
function within its class, although I think the wording should be
improved to make it clearer.

The virtual specifier shall only be
used only in declarationsthe initial declaration
of a non-static class member functions that appear within a
member-specification of a class definitionfunction;
see 10.3
class.virtual.

The explicit specifier shall be used only
in declarationsthe declaration of constructorsa constructor within aits class definition; see
12.3.1
class.conv.ctor.

283.
Template type-parameters are not syntactically type-names

A type-parameter defines its identifier to be a
type-name (if declared with class or
typename)

the grammar in 7.1.5.2
dcl.type.simple paragraph 1
says that a type-name is either a class-name, an
enum-name, or a typedef-name. The identifier
in a template type-parameter is none of those. One
possibility might be to equate the identifier with a
typedef-name instead of directly with a type-name,
which would have the advantage of not requiring parallel treatment
of the two in situations where they are treated the same (e.g.,
in elaborated-type-specifiers, see
issue 245). See also
issue 215.

Proposed resolution (Clark Nelson, March 2002):

In 14.1
temp.param paragraph 3,
change "A type-parameter defines its identifier to be a
type-name"
to "A type-parameter defines its identifier
to be a typedef-name"

In 7.1.5.3
dcl.type.elab paragraph 2,
change "If the identifier resolves to a typedef-name or a template
type-parameter"
to "If the identifier resolves to a typedef-name".

This has been consolidated with the edits for some other
issues. See N1376=02-0034.

516.
Use of signed in bit-field declarations

It is implementation-defined whether bit-fields and objects
of char type are represented as signed or unsigned
quantities. The signed specifier forces char
objects and bit-fields to be signed; it is redundant with other
integral types.

The last sentence in that quote is misleading w.r.t. bit-fields. The
first sentence in that quote is correct but incomplete.

Proposed fix: change the two sentences to read:

It is implementation-defined whether objects of char type are
represented as signed or unsigned quantities. The signed
specifier forces char objects signed; it is redundant with
other integral types except when declaring bit-fields (9.6
class.bit).

When multiple simple-type-specifiers are allowed, they can be
freely intermixed with other decl-specifiers in any
order. [Note: It is implementation-defined whether
bit-fields and objects of char type and certain
bit-fields (9.6
class.bit) are represented as signed
or unsigned quantities. The signed specifier
forces bit-fields andchar objects and
bit-fields to be signed; it is redundant with other integral
typesin other contexts. —end note]

172.
Unsigned int as underlying type of enum

According to 7.2
dcl.enum
paragraph 5,
the underlying type of an enum is an unspecified integral type,
which could potentially be unsigned int. The promotion
rules in 4.5
conv.prom
paragraph 2 say
that such an enumeration value used in an expression will be promoted
to unsigned int. This means that a conforming
implementation could give the value false for the
following code:

enum { zero };
-1 < zero; // might be false

This is counterintuitive. Perhaps the description of the underlying
type of an enumeration should say that an unsigned underlying type
can be used only if the values of the enumerators cannot be
represented in the corresponding signed type. This approach would
be consistent with the treatment of integral promotion of
bitfields (4.5
conv.prom
paragraph 3).

It is implementation-defined which integral type is used as
the underlying type for an enumeration except that the
underlying type shall not be larger than int unless
the value of an enumerator cannot fit in an int
or unsigned intneither int nor
unsigned int can represent all the enumerator values.
Furthermore, the underlying type shall not be an unsigned
type if the corresponding signed type can represent all the
enumerator values.

It was noted that 4.5
conv.prom promotes unsigned
types smaller than int to (signed) int. The
signedness chosen by an implementation for small underlying types
is therefore unobservable, so the last sentence of the proposed
resolution above should apply only to int and larger
types. This observation also prompted discussion of an
alternative approach to resolving the issue, in which the
bmin and bmax of the enumeration
would determine the promoted type rather than the underlying type.

An rvalue of type wchar_t (3.9.1
basic.fundamental)
or an enumeration type
(7.2
dcl.enum) can be converted to an rvalue of
the first of the following
types that can represent all the values of its underlying type:
int, unsigned int, long, or unsigned long.

to

An rvalue of type wchar_t (3.9.1
basic.fundamental)
can be converted to an rvalue of the first of the following
types that can represent all the values of its underlying type:
int, unsigned int, long, or unsigned long.
An rvalue of an enumeration type (7.2
dcl.enum)
can be converted to an rvalue of the first of the following
types that can represent all the values of the enumeration (i.e., the
values in the range bmin to bmax as
described in 7.2
dcl.enum):
int, unsigned int, long, or unsigned long.

540.
Propagation of cv-qualifiers in reference-to-reference collapse

[Voted into the WP at the October, 2006 meeting as part of
paper J16/06-0188 = WG21 N2118.]

The resolution of issue 106 specifies
that an attempt to create a type “reference to cv1T,” where T is a typedef or template parameter
of the type “reference to cv2S,” actually
creates the type “reference to cv12S,”
where cv12 is the union of the two sets of cv-qualifiers.

One objection that has been raised to this resolution is that it
is inconsistent with the treatment of cv-qualification and references
specified in 8.3.2
dcl.ref paragraph 1, which says
that cv-qualifiers applied to a typedef or template argument that is
a reference type are ignored. For example:

In fact, however, these two declarations are quite different. In the
declaration of r1, const applies to a
“top-level” reference, while in the declaration of t2,
it occurs under a reference. In general, cv-qualifiers that appear under
a reference are preserved, even if the type appears in a context in which
top-level cv-qualification is removed, for example, in determining the
type of a function from parameter types (8.3.5
dcl.fct
paragraph 3) and in template argument deduction
(14.8.2.1
temp.deduct.call paragraph 2).

Another objection to the resolution is that type composition gives
different results in a single declaration than it does when separated
into two declarations. For example:

The initial motivation for the propagation of cv-qualification
during reference-to-reference collapse was to prevent inadvertent
loss of cv-qualifiers in contexts in which it could make a
difference. For example, if the resolution were changed to discard,
rather than propagate, embedded cv-qualification, overload
resolution could surprisingly select a non-const version of a member
function:

On the other hand, those who would like to see the resolution
changed to discard embedded cv-qualifiers observe that these examples
are too simple to be representative of real-world code. In general,
it is unrealistic to expect that a template written with non-reference
type parameters in mind will automatically work correctly with
reference type parameters as a result of applying the issue 106
resolution. Instead, template metaprogramming allows the template
author to choose explicitly whether cv-qualifiers are propagated or
dropped, according to the intended use of the template, and it is more
important to respect the reasonable intuition that a declaration
involving a template parameter will not change the type that the
parameter represents.

As a sample of real-world code, tr1::tuple was examined.
In both cases — the current resolution of issue 106 and one in
which embedded cv-qualifiers were dropped — some metaprogramming
was required to implement the intended interface, although the version
reflecting the revised resolution was somewhat simpler.

Notes from the October, 2005 meeting:

The consensus of the CWG was that the resolution of
issue 106 should be revised not to
propagate embedded cv-qualification.

Note (February, 2006):

The wording included in the rvalue-reference paper,
J16/06-0022 = WG21 N1952, incorporates changes intended to implement
the October, 2005 consensus of the CWG.

11.
How do the keywords typename/template interact with using-declarations?

When the rules for typename and the similar use of template
were decided, we chose to require that they be used at every reference.
The way to avoid typename at every use is to declare a typedef;
then the typedef name itself is known to be a type. For using-declarations,
we decided that they do not introduce new declarations but rather are aliases
for existing declarations, like symbolic links. This makes it unclear whether
the declaration "X c;" above should be well-formed, because there
is no new name declared so there is no declaration with a "this is a type"
attribute. (The same problem would occur with the template keyword
when a member template of a dependent class is used). I think these are
the main options:

Continue to allow typename in using-declarations, and template
(for member templates) too. Attach the "is a type" or "is a template"
attribute to the placeholder name which the using-declaration "declares"

Disallow typename and template in using-declarations
(just as class-keys are disallowed now). Allow typename
and template before unqualified names which refer to dependent
qualified names through using-declarations.

Document that this is broken.

Suggested Resolution:

The core WG already resolved this issue according to (1), but the wording
does not seem to have been added to the standard. New wording needs to
be drafted.

Issue 2:

Either way, one more point needs clarification. If the first option
is adopted:

When "g" is instantiated, the two declarations of X are compatible
(7.3.3
namespace.udecl
paragraph 10).
But there is no way to know this when the definition of "g" is
compiled. I think this case should be ill-formed under the first option.
(It cannot happen under the second option.) If the second option is adopted:

Again, the instantiation would work but there is no way to know that in
the template definition. I think this case should be ill-formed under the
second option. (It would already be ill-formed under the first option.)

From John Spicer:

The "not a new declaration" decision is more of a guiding principle
than a hard and fast rule. For example, a name introduced in a
using-declaration
can have different access than the original declaration.

Like symbolic links, a using-declaration can be viewed as a declaration
that declares an alias to another name, much like a typedef.

In my opinion, "X c;" is already well-formed. Why would we permit
typename to be used in a using-declaration if not to permit this
precise usage?

In my opinion, all that needs to be done is to clarify that the "typeness"
or "templateness" attribute of the name referenced in the using-declaration
is attached to the alias created by the using-declaration. This is
solution #1.

Tentative Resolution:

The rules for multiple declarations with the same name in the same scope
should treat a using-declaration which names a type as a typedef,
just as a typedef of a class name is treated as a class declaration. This
needs drafting work. Also see Core issue 36.

Rationale (04/99): Any semantics associated with the
typename keyword in using-declarations should be considered an
extension.

Notes from the April 2003 meeting:

This was reopened because we are now considering extensions again.
We agreed that it is desirable for the typename to be "sticky" on
a using-declaration, i.e., references to the name introduced by
the using-declaration are known to be type names without the use
of the typename keyword (which can't be specified on an unqualified
name anyway, as of now). The related issue with the template keyword
already has a separate issue 109.

Issue 2 deals with the "struct hack."
There is an example in 7.3.3
namespace.udecl paragraph 10
that shows a use of using-declarations to import two names that
coexist because of the "struct hack." After some deliberation,
we decided that the template-dependent using-declaration case is
different enough that we did not have to support the "struct hack"
in that case.
A name introduced in such a case is like a typedef, and no other
hidden type can be accessed through an elaborated type specifier.

258.
using-declarations and cv-qualifiers

When a using-declaration brings names from a base class into a
derived class scope, member functions in the derived class override
and/or hide member functions with the same name and parameter types in
a base class (rather than conflicting).

Note that this description says nothing about the cv-qualification
of the hiding and hidden member functions. This means, for instance,
that a non-const member function in the derived class hides a const
member function with the same name and parameter types instead of
overloading it in the derived class scope. For example,

If a virtual member function vf is declared in a class
Base and in a class Derived, derived directly or
indirectly from Base, a member function vf with the
same name and same parameter list as Base::vf is declared,
then Derived::vf is also virtual (whether or not it is so
declared) and it overrides Base::vf.

Notes on the 04/01 meeting:

The hiding and overriding should
be on the basis of the function signature, which includes any
cv-qualification on the function.

When a using-declaration brings names from a base class into a
derived class scope, member functions in the derived class override and/or
hide member functions with the same name and parameter types in a base
class (rather than conflicting).

to read:

When a using-declaration brings names from a base class into a
derived class scope, member functions and member function templates in
the derived class override and/or hide member functions and member
function templates with the same name, parameter-type-list
(8.3.5
dcl.fct),
and cv-qualification in a base class (rather than conflicting).

If a virtual member function vf is declared in a class
Base and in a class Derived, derived directly or
indirectly from Base, a member function vf with the
same name and same parameter list as Base::vf is declared,
then Derived::vf is also virtual (whether or not it is so
declared) and it overrides Base::vf.

to read:

If a virtual member function vf is declared in a class
Base and in a class Derived, derived directly or
indirectly from Base, a member function vf with the
same name, parameter-type-list (8.3.5
dcl.fct), and
cv-qualification as Base::vf
is declared, then Derived::vf is also virtual (whether or not
it is so declared) and it overrides Base::vf.

Several popular compilers give an error on this, but there
doesn't seem to be anything in 7.3.3
namespace.udecl that
prohibits it. It should be noted that the user can get the same
effect by using a namespace alias:

namespace my_namespace2 = ::my_namespace::my_namespace2;

Notes from the March 2004 meeting:

We agree that it should be an error.

Proposed resolution (October, 2004):

Add the following as a new paragraph after 7.3.3
namespace.udecl paragraph 5:

A using-declaration shall not name a namespace;

4.
Does extern "C" affect the linkage of function names with internal linkage?

At most one of a set of overloaded functions with a particular name
can have C linkage.

Does this apply to static functions as well? For example, is the following
well-formed?

extern "C" {
static void f(int) {}
static void f(float) {}
};

Can a function with internal linkage "have C linkage" at all (assuming
that phrase means "has extern "C" linkage"), for how can a function be
extern "C" if it's not extern? The function type can have extern
"C" linkage — but I think that's independent of the linkage of the function
name. It should be perfectly reasonable to say, in the example above,
that extern "C" applies only to the types of f(int) and f(float),
not to the function names, and that the rule in 7.5
dcl.link
paragraph 6 doesn't apply.

Suggested resolution: The extern "C" linkage specification applies
only to the type of functions with internal linkage, and therefore some
of the rules that have to do with name overloading don't apply.

Proposed Resolution:

The intent is to distingush implicit linkage from explicit linkage
for both name linkage and language (function type) linkage. (It might
be more clear to use the terms name linkage and type linkage
to distinguish these concepts. A function can have a name with one
kind of linkage and a type with a different kind of linkage. The
function itself has no linkage: it has no name, only the declaration has
a name. This becomes more obvious when you consider function pointers.)

The tentatively agreed proposal is to apply implicit linkage to names
declared in brace-enclosed linkage specifications and to non-top-level
names declared in simple linkage specifications; and to apply explicit
linkage to top-level names declared in simple linkage specifications.

The language linkage of any function type formed through a function
declarator is that of the nearest enclosing linkage-specification.
For purposes of determining whether the declaration of a namespace-scope
name matches a previous declaration, the language linkage portion of the
type of a function declaration (that is, the language linkage of the function
itself, not its parameters, return type or exception specification) is
ignored.

For a linkage-specification using braces, i.e.

externstring-literal{declaration-seqopt}

the linkage of any declaration of a namespace-scope name (including local
externs) which is not contained in a nested linkage-specification,
is not declared to have no linkage (static), and does not match a previous
declaration is given the linkage specified in the string-literal.
The language linkage of the type of any function declaration of a namespace-scope
name (including local externs) which is not contained in a nested linkage-specification
and which is declared with function declarator syntax is the same as that
of a matching previous declaration, if any, else is specified by string-literal.

For a linkage-specification without braces, i.e.

externstring-literaldeclaration

the linkage of the names declared in the top-level declarators of declaration
is specified by string-literal; if this conflicts with the linkage
of any matching previous declarations, the program is ill-formed. The language
linkage of the type of any top-level function declarator is specified by
string-literal; if this conflicts with the language linkage of the
type of any matching previous function declarations, the program is ill-formed.
The effect of the linkage-specification on other (non top-level)
names declared in declaration is the same as that of the brace-enclosed
form.

extern "C++" void f(void(*fp)()); // error - linkage of "f" does not
// match previous declaration
// (linkage of function type used in
// parameter is still "C" and is not
// by itself ill-formed)
extern "C" void g(void(*fp)()); // error - linkage of "g" does not
// match previous declaration
// (linkage of function type used in
// parameter is still "C++" and is not
// by itself ill-formed)

That is, non-top-level declarators get their linkage from matching
declarations, if any, else from the nearest enclosing linkage
specification. (As already described, top-level declarators in a
brace-enclosed linkage specification get the linkage from matching
declarations, if any, else from the linkage specifcation; while
top-level declarators in direct linkage specifications get their
linkage from that specification.)

Mike Miller: This is a pretty significant change from the
current specification, which treats the two forms of language linkage
similarly for most purposes. I don't understand why it's desirable to
expand the differences.

It seems very unintuitive to me that you could have a top-level
declaration in an extern "C" block that would not
receive "C" linkage.

In the current standard, the statement in
7.5
dcl.link paragraph 4 that

the specified language linkage applies to the function types of all
function declarators, function names, and variable names introduced by
the declaration(s)

both "C++" declarations would be well-formed, declaring an
overloaded version of f that takes a pointer to a "C++"
function as a parameter. I wouldn't expect that either declaration
would be a redeclaration (valid or invalid) of the "C" version of
f.

Bill Gibbons: The potential difficulty is the matching
process and the handling of deliberate overloading based on language
linkage. In the above examples, how are these two declarations
matched:

given that the linkage that is part of fp1 is "C" while the linkage (prior
to the matching process) that is part of fp2 is "C++"?

The proposal is that the linkage which is part of the parameter type is not
determined until after the match is attempted. This almost always correct
because you can't overload "C" and "C++" functions; so if the function names
match, it is likely that the declarations are supposed to be the
same.

Mike Miller: This seems like more trouble than it's worth.
This comparison of function types ignoring linkage specifications is,
as far as I know, not found anywhere in the current standard. Why do
we need to invent it?

Bill Gibbons: It is possible to construct pathological cases where this fails, e.g.

In this case the two function declarations refer to different
functions.

Mike Miller: This seems pretty strange to me. I think it
would be simpler to determine the type of the parameter based on the
containing linkage specification (implicitly "C++") and require a
typedef if the user wants to override the default behavior. For
example:

In a linkage-specification, the specified language
linkage applies to the function types of all function
declarators, function names, and variable names
introduced by the declaration(s).

to

In a linkage-specification, the specified language
linkage applies to the function types of all function
declarators, function names with external linkage, and
variable names with external linkage declared within
the linkage-specification.

Except for functions with internal linkage, a function
first declared in a linkage-specification behaves as a
function with external linkage. [Example:

extern "C" double f();
static double f(); // error

is ill-formed (7.1.1
dcl.stc). ] The form of
linkage-specification that contains a braced-enclosed
declaration-seq does not affect whether the contained
declarations are definitions or not (3.1
basic.def); the
form of linkage-specification directly containing a
single declaration is treated as an extern specifier
(7.1.1
dcl.stc) for the purpose of determining whether
the contained declaration is a definition. [Example:

extern "C" int i; // declaration
extern "C" {
int i; // definition
}

—end example] A linkage-specification directly
containing a single declaration shall not specify a
storage class. [Example:

extern "C" static void f(); // error

—end example]

to

A declaration directly contained in a
linkage-specification is treated as if it contains the
extern specifier (7.1.1
dcl.stc) for the purpose of
determining the linkage of the declared name and
whether it is a definition. Such a declaration shall
not specify a storage class. [Example:

29.
Linkage of locally declared functions

[Moved to DR at October 2002 meeting. This was incorrectly
marked as having DR status between 4/01 and 4/02. It was overlooked
when issue 4 was moved to DR at the 4/01
meeting; this one should have been moved as well, because it's resolved
by the changes there.]

Consider the following:

extern "C" void foo()
{
extern void bar();
bar();
}

Does "bar()" have "C" language linkage?

The ARM is explicit and says

A linkage-specification for a function also applies to functions
and objects declared within it.

The DIS says

In a linkage-specification, the specified language linkage
applies to the function types of all function declarators, function names,
and variable names introduced by the declaration(s).

In
3.8
basic.life
paragraph 5, replace "malloc" by
"std::malloc" in the example code and insert
"#include <cstdlib>" at the beginning of the example
code.

In
3.9
basic.types
paragraph 2, replace "memcpy" by
"std::memcpy" (the only instance in the footnote and both
instances in the example) and replace "memmove" by
"std::memmove" in the footnote (see also
issue 43).

In
3.9
basic.types
paragraph 3, replace "memcpy" by
"std::memcpy", once in the normative text and once in the
example (see also issue 43).

In
15.4
except.spec
paragraph 9, replace "unexpected" by
"std::unexpected" and "terminate" by
"std::terminate".

In
15.5
except.special
paragraph 1, replace "terminate" by
"std::terminate" and "unexpected" by
"std::unexpected".

In the heading of
15.5.1
except.terminate,
replace "terminate" by "std::terminate".

In
15.5.1
except.terminate
paragraph 1, footnote in the first bullet, replace
"terminate" by "std::terminate". In the
same paragraph, fifth bullet, replace "atexit" by
"std::atexit". In the same paragraph, last bullet,
replace "unexpected_handler" by
"std::unexpected_handler".

—end example]
[Note: an "array of Ncv-qualifier-seqT"
has cv-qualified type; such
an array has internal linkage unless explicitly declared extern
(7.1.5.1
dcl.type.cv
)
and must be initialized as specified in
8.5
dcl.init
. ]

The Note appears to contradict the sentence that precedes it.

Mike Miller:
I disagree; all it says is that whether the qualification on
the element type is direct ("const int x[5]") or indirect
("const A CA"), the array itself is qualified in the same way
the elements are.

A compound type (3.9.2
basic.compound
)
is not cv-qualified by the cv-qualifiers (if
any) of the types from which it is compounded. Any cv-qualifiers
applied to an array type affect the array element type, not the
array type (8.3.4
dcl.array
)."

Cv-qualifiers applied to an array type attach to the
underlying element type, so the notation "cvT,"
where T is
an array type, refers to an array whose elements are
so-qualified. Such array types can be said to be more (or
less) cv-qualified than other types based on the cv-qualification
of the underlying element types.

I think this says essentially the same thing as
8.3.4
dcl.array
paragraph 1 and
its note: the qualification of an array is (bidirectionally)
equivalent to the qualification of its members.

Mike Ball:
I find this a very far reach. The text in
8.3.4
dcl.array
is essentially that which
is in the C standard (and is a change from early versions of C++).
I don't see any justification at all for the bidirectional equivalence.
It seems to me that
the note is left over from the earlier version of the language.

Steve Clamage: Finally, the Note seems to say that the declaration

volatile char greet[6] = "Hello";

gives "greet" internal linkage, which makes no sense.

Have I missed something, or should that Note be entirely removed?

Mike Miller:
At least the wording in the note
should be repaired not to indicate that volatile-qualification
gives an array internal linkage. Also, depending on how the
discussion goes, either the wording in
3.9.3
basic.type.qualifier
paragraph 2
or in paragraph 5 needs
to be amended to be consistent regarding whether an array type
is considered qualified by the qualification of its element
type.

Steve Adamczyk pointed out that
the current state of affairs resulted from the need to handle
reference binding consistently. The wording is intended to define
the question, "Is an array type cv-qualified?" as being equivalent
to the question, "Is the element type of the array cv-qualified?"

Proposed resolution (10/00):

Replace the portion of the note in 8.3.4
dcl.array
paragraph 1 reading

such an array has internal linkage unless explicitly declared
extern (7.1.5.1
dcl.type.cv) and must be
initialized as specified in 8.5
dcl.init.

140.
Agreement of parameter declarations

All declarations for a function with a given parameter list shall
agree exactly both in the type of the value returned and in the
number and type of parameters.

It is not clear what this requirement means with respect to a pair
of declarations like the following:

int f(const int);
int f(int x) { ... }

Do they violate this requirement? Is x const in the body of
the function declaration?

Tom Plum:
I think the FDIS quotation means that the pair of decls are valid.
But it doesn't clearly answer whether
x is const inside the function definition.
As to intent, I know the intent was that if the function
definition wants to specify that x is const,
the const must appear specifically in the defining
decl, not just on some decl elsewhere.
But I can't prove that intent from the drafted words.

Mike Miller:
I think the intent was something along the
following lines:

Two function declarations denote the same entity if
the names are the same and the function signatures
are the same. (Two function declarations with C
language linkage denote the same entity if the names
are the same.) All declarations of a given function
shall agree exactly both in the type of the value
returned and in the number and type of parameters;
the presence or absence of the ellipsis is considered
part of the signature.

(See 3.5
basic.link
paragraph 9.
That paragraph talks about names in different
scopes and says that function references are the same if the
"types are identical for purposes of overloading," i.e., the
signatures are the same. See also
7.5
dcl.link
paragraph 6
regarding C language
linkage, where only the name is required to be the same for
declarations in different namespaces to denote the same
function.)

According to this paragraph, the type of a parameter is
determined by considering its decl-specifier-seq and
declarator
and then applying the array-to-pointer and function-to-pointer
adjustments. The cv-qualifier and storage class adjustments
are performed for the function type but not for the parameter
types.

If my interpretation of the intent of the second sentence of
the paragraph is correct, the two declarations in the example
violate that restriction — the parameter types are not the
same, even though the function types are. Since there's no
dispensation mentioned for "no diagnostic required," an
implementation presumably must issue a diagnostic in this
case. (I think "no diagnostic required" should be stated if
the declarations occur in different translation units —
unless there's a blanket statement to that effect that I have
forgotten?)

In the third bullet of 3.5
basic.link paragraph 9
change
"the function types are identical for the purposes of overloading"
to
"the parameter-type-lists of the functions (8.3.5
dcl.fct) are identical."

In the sub-bullets of the third bullet of
5.2.5
expr.ref paragraph 4, change all four occurrences
of
"function of (parameter type list)"
to
"function of parameter-type-list."

All declarations for a function with a given parameter list shall agree
exactly both in the type of the value returned and in the number and
type of parameters; the presence or absence of the ellipsis is
considered part of the function type.

to

All declarations for a function shall
agree exactly in both the return type and the
parameter-type-list.

The resulting list of transformed parameter types is the function's
parameter type list.

to

The resulting list of transformed parameter types and the presence or
absence of the ellipsis is the function's parameter-type-list.

In 8.3.5
dcl.fct paragraph 4, change
"the parameter type list"
to
"the parameter-type-list."

In the second bullet of 13.1
over.load paragraph 2,
change all occurrences of "parameter types" to
"parameter-type-list."

In 13.3
over.match paragraph 1, change "the types of
the parameters" to "the parameter-type-list."

In the last sub-bullet of the third bullet of
13.3.1.2
over.match.oper paragraph 3, change "parameter type
list" to "parameter-type-list."

Note, 7 Sep 2001:

Editorial changes while putting in issue 147
brought up the fact that injected-class-name is not a syntax term and
therefore perhaps shouldn't be written with hyphens. The same can be said of
parameter-type-list.

The effect of a cv-qualifier-seq in a function declarator is
not the same as adding cv-qualification on top of the function type,
i.e., it does not create a cv-qualified function type. In fact, if at
any time in the determination of a type a cv-qualified function type
is formed, the program is ill-formed. [Example:

typedef void F();
struct S {
const F f; // ill-formed
};

-- end example]

by

The effect of a cv-qualifier-seq in a function declarator is
not the same as adding cv-qualification on top of the function type.
In the latter case, the cv-qualifiers are ignored. [Example:

Strike the last bulleted item in 14.8.2
temp.deduct
paragraph 2, which reads

Attempting to create a cv-qualified function type.

Nathan Sidwell comments (18 Dec 2001
):
The proposed resolution simply states attempts to add cv qualification
on top of a function type are ignored. There is no mention of whether
the function type was introduced via a typedef or template type parameter.
This would appear to allow

void (const *fptr) ();

but, that is not permitted by the grammar. This is inconsistent
with the wording of adding cv qualifiers to a reference type, which does
mention typedefs and template parameters, even though

int &const ref;

is also not allowed by the grammar.

Is this difference intentional? It seems needlessly confusing.

Notes from 4/02 meeting:

Yes, the difference is intentional. There is no way to add cv-qualifiers
other than those cases.

Notes from April 2003 meeting:

Nathan Sidwell pointed out that some libraries use the inability to
add const to a type T as a way of testing that T is a function
type. He will get back to us if he has a proposal for a change.

136.
Default arguments and friend declarations

For non-template functions, default arguments can be added in later
declarations of a function in the same scope. Declarations in
different scopes have completely distinct sets of default arguments.
That is, declarations in inner scopes do not acquire default arguments
from declarations in outer scopes, and vice versa.

It is unclear how this wording applies to friend function declarations.
For example,

Does the declaration at #2 acquire the default argument from #1, and
does the one at #3 acquire the default arguments from #2?

There are several related questions involved with this issue:

Is the friend
declaration in the scope of class C or in the surrounding namespace
scope?

Mike Miller:
8.3.6
dcl.fct.default
paragraph 4
is speaking about the lexical location of the
declaration...
The friend declaration occurs in a different declarative region
from the declaration at #1, so I would read [this paragraph] as saying that it
starts out with a clean slate of default arguments.

Bill Gibbons:
Yes. It occurs in a different region, although it declares a name
in the same region (i.e. a redeclaration). This is the same as with
local externs and is intended to work the same way. We decided that
local extern declarations cannot add (beyond the enclosing block) new
default arguments, and the same should apply to friend declarations.

John Spicer:
The question is whether [this paragraph]
does (or should) mean declarations that appear in the same lexical
scope or declarations that declare names in the same scope. In my opinion,
it really needs to be the latter. It seems somewhat paradoxical to say
that a friend declaration declares a function in namespace scope yet the
declaration in the class still has its own attributes. To make that work
I think you'd have to make friends more like block externs that really do
introduce a name into the scope in which the declaration is contained.

Should default arguments be permitted in friend function
declarations, and what effect should they have?

Bill Gibbons:
In the absence of a declaration visible in class scope to which
they could be attached, default arguments on friend declarations
do not make sense.
[They should be] ill-formed, to prevent surprises.

In other words, a function first declared in a friend declaration must be
permitted to have default arguments and those default arguments must be
usable when the function is found by argument dependent lookup. The reason
that this is important is that it is common practice to define functions
in friend declarations in templates, and that definition is the only place
where the default arguments can be specified.

What restrictions should be placed on default argument usage with
friend declarations?

John Spicer:
We want to avoid instantiation side effects. IMO, the way to do this
would be to prohibit a friend declaration from providing default arguments
if a declaration of that function is already visible.
Once a function has had a default specified in a friend
declaration it should not be possible to add defaults in another declaration
be it a friend or normal declaration.

Mike Miller:
The position that seems most reasonable to me is to
allow default arguments in friend declarations to be used in
Koenig lookup, but to say that they are
completely unrelated to default arguments in declarations in
the surrounding scope; and to forbid use of a default argument
in a call if more than one declaration in the overload set has
such a default, as in the proposed resolution for
issue 1.

5.
CV-qualifiers and type conversions

The description of copy-initialization in 8.5
dcl.init
paragraph 14 says:

If the destination type is a (possibly cv-qualified) class type:
...

Otherwise (i.e. for the remaining copy-initialization cases), user-defined
conversion sequences that can convert from the source type to the destination
type or (when a conversion function is used) to a derived class thereof
are enumerated ... if the function is a constructor, the call initializes
a temporary of the destination type. ...

Should "destination type" in this last bullet refer to "cv-unqualified
destination type" to make it clear that the destination type excludes any
cv-qualifiers? This would make it clearer that the following example is
well-formed:

The temporary created with the conversion function is an lvalue of type
B. If the temporary must have the cv-qualifiers of the destination
type (i.e. const) then the copy-constructor for A cannot be called
to create the object of type A from the lvalue of type const
B. If the temporary has the cv-qualifiers of the result type of the
conversion function, then the copy-constructor for A can be called
to create the object of type A from the lvalue of type const
B. This last outcome seems more appropriate.

Steve Adamczyk:

Because of late changes to this area, the
relevant text is now the third sub-bullet of the fourth bullet of
8.5
dcl.init paragraph 14:

Otherwise (i.e., for the remaining copy-initialization cases),
user-defined conversion sequences that can convert from the source
type to the destination type or (when a conversion function is used)
to a derived class thereof are enumerated...
The function selected is called with the initializer expression as its
argument; if the function is a constructor, the call initializes a
temporary of the destination type. The result of the call (which is
the temporary for the constructor case) is then used to
direct-initialize, according to the rules above, the object that is
the destination of the copy-initialization.

The issue still remains whether the wording should refer to "the
cv-unqualified version of the destination type." I think it
should.

Notes from 10/00 meeting:

The original example does not illustrate the remaining
problem. The following example does:

78.
Section 8.5 paragraph 9 should state it only applies to non-static objects

If no initializer is specified for an object, and the object
is of (possibly cv-qualified) non-POD class type (or array thereof), the
object shall be default-initialized; if the object is of const-qualified
type, the underlying class type shall have a user-declared default constructor.
Otherwise, if no initializer is specified for an object, the object and
its subobjects, if any, have an indeterminate initial value; if the object
or any of its subobjects are of const-qualified type, the program is ill-formed.

It should be made clear that this paragraph does not apply to static objects.

According to 8.5
dcl.init
paragraph
14, the initialization of a is performed in two steps.
First, a temporary of type A is created using A::A(const
B&). Second, the resulting temporary is used to
direct-initialize a using A::A(A&).

The second step requires binding a reference to non-const to the
temporary resulting from the first step. However,
8.5.3
dcl.init.ref
paragraph 5 requires
that such a reference be bound only to lvalues.

It is not clear from 3.10
basic.lval
whether the temporary created in the process of copy-initialization
should be treated as an lvalue or an rvalue. If it is an lvalue, the
example is well-formed, otherwise it is ill-formed.

Proposed resolution (04/01):

In 8.5
dcl.init paragraph 14, insert
the following after "the call initializes a temporary of
the destination type":

277.
Zero-initialization of pointers

The intent of 8.5
dcl.init paragraph 5 is that
pointers that are zero-initialized will contain a null pointer
value. Unfortunately, the wording used,

...set to the value of 0 (zero) converted to T

does not match the requirements for creating a null pointer
value given in 4.10
conv.ptr paragraph 1:

A null pointer constant is an integral constant expression
(5.19
expr.const) rvalue of integer type that evaluates to
zero. A null pointer constant can be converted to a pointer type; the
result is the null pointer value of that type...

The problem is that the "value of 0" in the description of
zero-initialization is not specified to be an integral constant
expression. Nonconstant expressions can also have the value 0,
and converting a nonconst 0 to pointer type need not result in
a null pointer value.

...set to the value 0 (zero), taken as an integral constant
expression, converted to T; [footnote: as
specified in 4.10
conv.ptr, converting an integral
constant expression whose value is 0 to a pointer type results
in a null pointer value.]

302.
Value-initialization and generation of default constructor

We've been looking at implementing value-initialization. At one point,
some years back, I remember Bjarne saying that something like X() in
an expression should produce an X object with the same value one would
get if one created a static X object, i.e., the uninitialized members
would be zero-initialized because the whole object is initialized
at program startup, before the constructor is called.

The formulation for default-initialization that made it into TC1 (in
8.5
dcl.init)
is written a little differently (see issue 178),
but I had always
assumed that it would still be a valid implementation to zero the whole
object and then call the default constructor for the troublesome
"non-POD but no user-written constructor" cases.

That almost works correctly, but I found a problem case:

struct A {
A();
~A();
};
struct B {
// B is a non-POD with no user-written constructor.
// It has a nontrivial generated constructor.
const int i;
A a;
};
int main () {
// Value-initializing a "B" doesn't call the default constructor for
// "B"; it value-initializes the members of B. Therefore it shouldn't
// cause an error on generation of the default constructor for the
// following:
new B();
}

If the definition of the B::B() constructor is generated, an error
is issued because the const member "i" is not initialized. But the
definition of value-initialization doesn't require calling the
constructor, and therefore it doesn't require generating it, and
therefore the error shouldn't be detected.

So this is a case where zero-initializing and then calling the constructor
is not equivalent to value-initializing, because one case generates
an error and the other doesn't.

This is sort of unfortunate, because one doesn't want to generate all
the required initializations at the point where the "()"
initialization
occurs. One would like those initializations to be packaged in a
function, and the default constructor is pretty much the function
one wants.

I see several implementation choices:

Zero the object, then call the default generated constructor. This
is not valid unless the standard is changed to say that the default
constructor might be generated for value-initialization cases like the
above (that is, it's implementation-dependent whether the constructor
definition is generated). The zeroing operation can of course be
optimized, if necessary, to hit only the pieces of the object that
would otherwise be left uninitialized. An alternative would be
to require generation of the constructor for value-initialization
cases, even if the implementation technique doesn't call the
constructor at that point. It's pretty likely that the constructor
is going to have to be generated at some point in the program anyway.

Make a new value-initialization "constructor," whose body looks a
lot like the usual generated constructor, but which also zeroes other members.
No errors would be generated while generating this modified constructor,
because it generates code that does full initialization. (Actually,
it wouldn't guarantee initialization of reference members, and that
might be an argument for generating the constructor, in order to get that
error.) This is standard-conforming, but it destroys object-code
compatibility.

Variation on (1): Zero first, and generate the object code for the
default constructor when it's needed for value-initialization cases, but
don't issue any errors at that time. Issue the errors only if it turns
out the constructor is "really" referenced. Aside from the essential
shadiness of this approach, I fear that something in the generation
of the constructor will cause a template instantiation which will be
an abservable side effect.

Personally, I find option 1 the least objectionable.

Proposed resolution (10/01):

Add the indicated wording to the third-to-last sentence of
3.2
basic.def.odr pararaph 2:

A default constructor for a class is used by default initialization
or value initialization as specified in 8.5
dcl.init.

if T is a non-union class type without a user-declared constructor,
then every non-static data member and base-class component of T is
value-initialized.
[Footnote: Value-initialization for such a class object may be
implemented by zero-initializing the object and then calling the default
constructor.]

Add the indicated wording to the first sentence of
12.1
class.ctor paragraph 7:

An implicitly-declared default constructor for a class is implicitly
defined when it is used (3.2
basic.def.odr) to
create an object of its class type
(1.8
intro.object).

491.
Initializers for empty-class aggregrate members

An initializer for an aggregate member that is an empty class
shall have the form of an empty initializer-list {}.

This is overly constraining. There is no reason that the
following should be ill-formed:

struct S { };
S s;
S arr[1] = { s };

Mike Miller: The wording of 8.5.1
dcl.init.aggr paragraph 8 is unclear. “An aggregate
member” would most naturally mean “a member of an
aggregate.” In context, however, I think it must mean
“a member [of an aggregate] that is an aggregate”,
that is, a subaggregate. Members of aggregates need not
themselves be aggregates (cf paragraph 13 and 12.6.1
class.expl.init); it cannot be the case that an object of an
empty class with a user-declared constructor must be initialized
with {} when it is a member of an aggregate. This
wording should be clarified, regardless of the decision on
Nathan's point.

There is a place in the Standard where overload resolution is implied
but the way that a set of candidate functions is to be formed is
omitted. See below.

According to the Standard, when initializing a reference to a
non-volatile const class type (cv1T1) with an rvalue
expression (cv2T2) where cv1T1 is
reference compatible with cv2T2, the implementation
shall proceed in one of the following
ways (except when initializing the implicit object parameter of a
copy constructor) 8.5.3
dcl.init.ref paragraph 5 bullet 2
sub-bullet 1:

The reference is bound to the object represented by the rvalue (see
3.10
basic.lval) or to a sub-object within that object.

A temporary of type "cv1T2" [sic] is created, and a
constructor is called to copy the entire rvalue object into the temporary...

While the first case is quite obvious, the second one is a bit unclear
as it says "a constructor is called to copy the entire rvalue object
into the temporary" without specifying how the temporary is created --
by direct-initialization or by copy-initialization? As stated in DR
152, this can make a difference when the copy constructor is declared
as explicit. How should the set of candidate functions be formed? The
most appropriate guess is that it shall proceed as per
13.3.1.3
over.match.ctor.

Another detail worth of note is that in the draft version of the
Standard as of 2 December 1996 the second bullet read:

A temporary of type "cv1T2" [sic] is created, and a copy
constructor is called to copy the entire rvalue object into the
temporary...

J. Stephen Adamczyk replied that the reason for changing "a copy
constructor" to "a constructor" was to allow for member template
converting constructors.

However, the new wording is somewhat in conflict with the footnote #93
that says that when initializing the implicit object parameter of a
copy constructor an implementation must eventually choose the first
alternative (binding without copying) to avoid infinite recursion.
This seems to suggest that a copy constructor is always used for
initializing the temporary of type "cv1T2".

Furthermore, now that the set of candidate functions is not limited to
only the copy constructors of T2, there might be some unpleasant
consequences.
Consider a rather contrived sample below:

In this example the initialization of the temporary of type
'<TT>const std::auto_ptr<int>'
(to which 'ri' is meant to be subsequently bound)
doesn't fail, as it would had the approach with copy constructors been
retained, instead, a yet another temporary gets created as the
well-known sequence:

is called (assuming, of course, that the set of candidate functions is
formed as per 13.3.1.3
over.match.ctor). The second temporary
is transient and gets
destroyed at the end of the initialization. I doubt that this is the
way that the committee wanted this kind of reference binding to go.

Besides, even if the approach restricting the set of candidates to copy
constructors is restored, it is still not clear how the initialization
of the temporary (to which the reference is intended to be bound) is
to be performed -- using direct-initialization or copy-initialization.

Another place in the Standard that would benefit from a similar
clarification is the creation of an exception object, which is
delineated in 15.1
except.throw.

David Abrahams (February 2004):
It appears, looking at core 291, that there may be a need to tighten
up 8.5.3
dcl.init.ref/5.

Please see the attached example file, which demonstrates "move
semantics" in C++98. Many compilers fail to compile test 10 because
of the way 8.5.3/5 is interpreted. My problem with that
interpretation is that test 20:

typedef X const XC;
sink2(XC(X()));

does compile. In other words, it *is* possible to construct the const
temporary from the rvalue. IMO, that is the proper test.

8.5.3/5 doesn't demand that a "copy constructor" is used to copy the
temporary, only that a constructor is used "to copy the temporary".
I hope that when the language is tightened up to specify direct (or
copy initialization), that it also unambiguously allows the enclosed
test to compile. Not only is it, I believe, within the scope of
reasonable interpretation of the current standard, but it's an
incredibly important piece of functionality for library writers and
users alike.

Change paragraph 5, second bullet,
first sub-bullet, second sub-sub-bullet as follows:

A temporary of type "cv1T2" [sic] is created, and a
constructor is called to copy the entire rvalue object into the temporaryvia copy-initialization from the entire rvalue object.
The reference is bound to the temporary or to a sub-object
within the temporary.

The text immediately following that is changed as follows:

The constructor that would be used to make the copy shall be
callable whether or not the copy is actually done.The constructor and any conversion function that would be used in the
initialization shall be callable whether or not the temporary is
actually created.

Note, however, that the way the core working group is leaning
on issue 391 (i.e., requiring direct
binding) would make this change unnecessary.

391.
Require direct binding of short-lived references to rvalues

After some email exchanges with Rani Sharoni, I've come up
with the following proposal to allow reference binding to
non-copyable rvalues in some cases. Rationale and some
background appear afterwards.

---- proposal ----

Replace the section of 8.5.3
dcl.init.ref paragraph 5
that begins "If the initializer expression is an rvalue" with the
following:

If the initializer expression is an rvalue, with T2 a
class type, and ``cv1 T1'' is reference-compatible with
``cv2 T2,'' the reference is bound as follows:

If the lifetime of the reference does not extend
beyond the end of the full expression containing the
initializer expression, the reference is bound to
the object represented by the rvalue (see 3.10
basic.lval) or
to a sub-object within that object.

otherwise, the reference is bound in one of the
following ways (the choice is implementation-defined):

[... continues as before - the original wording applies
unchanged to longer-lived references]

---- rationale ----

The intention of the current wording is to provide the
implementation freedom to construct an rvalue of class
type at an arbitrary location and copy it zero or more
times before binding any reference to it.

The standard allows code to call a member function on an
rvalue of class type (in 5.2.5
expr.ref, I guess).
This means that
the implementation can be forced to bind the reference
directly, with no freedom to create any temporary
copies. e.g.

Forcing a direct binding in this way is possible wherever
the lifetime of the reference does not extend beyond the
containing full expression, since the reference returned
by the member function remains valid for this long.

As demonstrated above, existing implementations must
already be capable of constructing an rvalue of class
type in the "right" place the first time. Some compilers
already silently allow the direct binding of references
to non-copyable rvalues.

The change will not break any portable user code. It
would break any platform-specific user code that relies
on copies being performed by the particular
implementation.

---- background ----

The proposal is based on a recent discussion in this
group. I originally wanted to leave the implementation free
to copy the rvalue if there was a callable copy constructor,
and only have to bind directly if none was callable.
Unfortunately, a traditional compiler can't always tell
whether a function is callable or not, e.g. if the copy
constructor is declared but not defined. Rani pointed this
out in an example, and suggested that maybe trivial copy
constructors should still be allowed (by extension, maybe
wherever the compiler can determine callability). I've gone
with this version because it's simpler, and I also figure
the "as if" rule gives the compiler some freedom with POD
types anyway.

Notes from April 2003 meeting:

We agreed generally with the proposal. We were unsure about the
need for the restriction regarding long-lived references.
We will check with the proposer about that.

Jason Merrill points out that the test case in
issue 86 may be a case where we
do not want to require direct binding.

Further information from Rani Sharoni (April 2003):

I wasn't aware about the latest suggestion of Raoul as it appears in
core issue 391. In our discussions we tried to formulate a different
proposal.

The rational, as we understood, behind the implementation freedom to
make an extra copying (8.5.3/5/2/12) of the rvalue is to allow return
values in registers which on some architectures are not addressable.
The example that Raoul and I presented shows that this implementation
freedom is not always possible since we can "force" the rvalue to be
addressable using additional member function (by_ref). The example
only works for short lived rvalues and this is probably why Raoul
narrow the suggestion.

I had different rational which was related to the implementation of
conditional operator in VC. It seems that when conditional operator is
involved VC does use an extra copying when the lifetime of the
temporary is extended:

I don't know what the consideration behind the VC implementation was
(I saw open bug on this issue) but it convinced me to narrow the
suggestion.

IMHO such limitation seems to be too strict because it might limit the
optimizer since returning class rvalues in registers might be useful
(although I'm not aware about any implementation that actually does
it). My suggestion was to forbid the extra copying if the ctor is not
viable (e.g. A::A(A&) ). In this case the implementation "freedom"
doesn't exist (since the code might not compile) and only limits the
programmer freedom (e.g. Move Constructors -
http://www.cuj.com/experts/2102/alexandr.htm).

Core issue 291 is strongly related to
the above issue and I personally
prefer to see it resolved first. It seems that VC already supports the
resolution I prefer.

Notes from October 2003 meeting:

We ended up feeling that this is just one of a number of cases
of optimizations that are widely done by compilers
and allowed but not required by the standard. We don't see any
strong reason to require compilers to do this particular
optimization.

Notes from the March 2004 meeting:

After discussing issue 450, we
found ourselves reconsidering this, and we are now inclined to
make a change to require the direct binding in all cases,
with no restriction on long-lived references. Note that such
a change would eliminate the need for a change for
issue 291.

If the initializer expression is an rvalue, with T2 a class type,
and "cv1T1" is reference-compatible with
"cv2T2", the reference is bound to the
object represented by the rvalue (see 3.10
basic.lval) or to a sub-object within that object.in one of the following ways (the choice is
implementation-defined):

The reference is bound to the object represented by the
rvalue (see 3.10
basic.lval) or to a sub-object
within that object.

A temporary of type "cv1T2" [sic] is created, and a
constructor is called to copy the entire rvalue object into the
temporary. The reference is bound to the temporary or to a
sub-object within the temporary.

The constructor that would be used to make the copy shall be
callable whether or not the copy is actually done.
[Example:

struct A { };
struct B : public A { } b;
extern B f();
const A& rca = f (); // BoundEither bound to the A sub-object of the B rvalue,
// or the entire B object is copied and the reference
// is bound to the A sub-object of the copy

When you run this through 8.5.3
dcl.init.ref, you sort of end up
falling off the end of the standard's description of reference
binding. The standard says in the final bullet of paragraph 5
that an array temporary should be created and copy-initialized
from the rvalue array, which seems implausible.

I'm not sure what the right answer is. I think I'd be happy with
allowing the binding in this case. We would have to introduce
a special case like the one for class rvalues.

Notes from the March 2004 meeting:

g++ and EDG give an error. Microsoft (8.0 beta) and Sun accept
the example. Our preference is to allow the direct binding (no copy).
See the similar issue with class rvalues in
issue 391.

If the initializer expression is an rvalue, with T2 an
array type, and “cv1T1” is
reference-compatible with “cv2T2”,
the reference is bound to the object represented by the rvalue
(see 3.10
basic.lval).

175.
Class name injection and base name access

With class name injection, when a base class name is used in a derived
class, the name found is the injected name in the base class, not the
name of the class in the scope containing the base class.
Consequently, if the base class name is not accessible (e.g., because
is is in a private base class), the base class name cannot be used
unless a qualified name is used to name the class in the class or
namespace of which it is a member.

Without class name injection the following example is valid. With
class name injection, A is inaccessible in class C.

[Note: In a derived class, the lookup of a base class name
will find the injected-class-name instead of the name of the base
class in the scope in which it was declared. The
injected-class-name might be less accessible than the name of the
base class in the scope in which it was declared.] [Example:

273.
POD classes and operator&()

I think that the definition of a POD class in the current version
of the Standard is overly permissive in that it allows for POD classes
for which a user-defined operator function operator& may
be defined. Given that the idea behind POD classes was to achieve
compatibility with C structs and unions, this makes 'Plain old'
structs and unions behave not quite as one would expect them to.

In the C language, if x and y are variables of
struct or union type S that has a member m, the
following expression are allowed: &x, x.m, x
= y. While the C++ standard guarantees that if x and
y are objects of a POD class type S, the expressions
x.m, x = y will have the same effect as they would
in C, it is still possible for the expression &x to be
interpreted differently, subject to the programmer supplying an
appropriate version of a user-defined operator function
operator& either as a member function or as a non-member
function.

For any complete POD object type T, whether or not the object
holds a valid value of type T, the underlying bytes
(1.7
intro.memory) making up the object can be copied into
an array of char or unsigned char [footnote:
By using, for example, the library functions (17.4.1.2
lib.headers) memcpy or memmove]. If the
content of the array of char or unsigned char is
copied back into the object, the object shall subsequently hold its
original value. [Example:

#define N sizeof(T)
char buf[N];
T obj; // obj initialized to its original value
memcpy(buf, &obj, N);
// between these two calls to memcpy,
// obj might be modified
memcpy(&obj, buf, N);
// at this point, each subobject of obj of scalar type
// holds its original value

—end example]

Now, supposing that the complete POD object type T in the
example above is POD_bomb, and we cannot any more count on
the assertions made in the comments to the example. Given a standard
conforming implementation, the code will not even compile. And I see
no legal way of copying the contents of an object of a complete object
type POD_bomb into an array of char or unsigned
char with memcpy or memmove without making use
of the unary & operator. Except, of course, by means of
an ugly construct like:

The macro offsetof accepts a restricted set of type arguments
in this International Standard. type shall be a POD structure
or a POD union (clause 9
class). The result of
applying the offsetof macro to a field that is a static data
member or a function is undefined."

A consensus was forming around the idea of disallowing
operator& in POD classes when it was noticed that it is
permitted to declare global-scope operator& functions,
which cause the same problems. After more discussion, it was
decided that such functions should not be prohibited in POD classes,
and implementors should simply be required to "get the right answer"
in constructs such as offsetof and va_start that
are conventionally implemented using macros that use the "&"
operator. It was noted that one can cast the original operand to
char & to de-type it, after which one can use the
built-in "&" safely.

284.
qualified-ids in class declarations

Although 8.3
dcl.meaning requires that a declaration of
a qualified-id refer to a member of the specified namespace or
class and that the member not have been introduced by a
using-declaration, it applies only to names declared in a
declarator. It is not clear whether there is existing wording
enforcing the same restriction for qualified-ids in
class-specifiers and elaborated-type-specifiers or
whether additional wording is required. Once such wording is
found/created, the proposed resolution of
issue 275 must be modified accordingly.

Notes from 10/01 meeting:

The sentiment was that this should be required on class definitions, but
not on elaborated type specifiers in general (which are references, not
declarations). We should also make sure we consider explicit instantiations,
explicit specializations, and friend declarations.

When a nested-name-specifier is specified in a
class-head or in an
elaborated-type-specifier, the resulting qualified name
shall refer to a previously declared member of the class or namespace to
which the nested-name-specifier refers, and
the member shall not have been introduced by a
using-declaration in the scope of the class or namespace
nominated by the nested-name-specifier.

327.
Use of "structure" without definition

In 9
class paragraph 4, the first sentence says
"A structure is a class
definition defined with the class-keystruct".
As far as I know,
there is no such thing as a structure in C++; it certainly isn't
listed as one of the possible compound types in 3.9.2
basic.compound.
And defining structures opens the question of whether a forward
declaration is a structure or not. The parallel here with union
(which follows immediately) suggests that structures and classes are
really different things, since the same wording is used, and classes
and unions do have some real differences, which manifest themselves
outside of the definition. It also suggests that since one can't
forward declare union with class and vice versa, the same should
hold for struct and class -- I believe that the intent was that one
could use struct and class interchangeably in forward declaration.

Suggested resolution:

I suggest something like the following:

If a class is defined with the class-keyclass, its members and
base classes are private by default. If a class is defined with
the class-keystruct,
its members and base classes are public by
default. If a class is defined with the class-keyunion, its
members are public by default, and it holds only one data member
at a time. Such classes are called unions, and obey a number of
additional restrictions, see 9.5
class.union.

379.
Change "class declaration" to "class definition"

The ARM used the term "class declaration" to refer to what
would usually be termed the definition of the class. The standard
now often uses "class definition", but there are some surviving uses
of "class declaration" with the old meaning. They should be found
and changed.

A declaration is a definition unless it declares a function
without specifying the function's body
(8.4
dcl.fct.def), it contains the
extern specifier (7.1.1
dcl.stc) or
a linkage-specification
[Footnote: Appearing inside the braced-enclosed
declaration-seq in a linkage-specification does not
affect whether a declaration is a definition. --- end
footnote]
(7.5
dcl.link) and neither an
initializer nor a function-body, it declares a static
data member in a class declarationdefinition
(9.4
class.static), it is a class name
declaration (9.1
class.name), or it is
a typedef declaration
(7.1.3
dcl.typedef), a
using-declaration
(7.3.3
namespace.udecl), or a
using-directive
(7.3.4
namespace.udir).

A
typedef-name
that names a class is a
class-name
(9.1
class.name).
If a typedef-name is used following the class-key
in an elaborated-type-specifier
(7.1.5.3
dcl.type.elab) or in the
class-head
of a class declarationdefinition
(9
class),
or is used as the identifier in the declarator for a
constructor or destructor declaration
(12.1
class.ctor,
12.4
class.dtor), the program is ill-formed.

There shall be no references to references, no arrays of references,
and no pointers to references. The declaration of a reference shall
contain an initializer
(8.5.3
dcl.init.ref) except when
the declaration contains an explicit extern specifier
(7.1.1
dcl.stc), is a class member
(9.2
class.mem) declaration within a
class declarationdefinition, or is the
declaration of a parameter or a return type
(8.3.5
dcl.fct); see
3.1
basic.def.

The initializer can be omitted for a reference only in a parameter
declaration (8.3.5
dcl.fct), in the
declaration of a function return type, in the declaration of a class
member within its class declarationdefinition
(9.2
class.mem), and where the
extern specifier is explicitly used.

A class definitiondeclaration introduces the
class name into the scope where it is defineddeclared and hides any class, object, function, or other
declaration of that name in an enclosing scope
(3.3
basic.scope). If a class name
is declared in a scope where an object, function, or enumerator of the
same name is also declared, then when both declarations are in scope,
the class can be referred to only using an
elaborated-type-specifier
(3.4.4
basic.lookup.elab).

Static members obey the usual class member access rules (clause
11
class.access). When used
in the declaration of a class member, the static specifier
shall only be used in the member declarations that appear within the
member-specification of the class declarationdefinition.

A class can be defineddeclared within another
class. A class defineddeclared within
another is called a nested class. The name of a nested class
is local to its enclosing class. The nested class is in the scope of
its enclosing class. Except by using explicit pointers, references,
and object names, declarations in a nested class can use only type
names, static members, and enumerators from the enclosing class.

A class can be defineddeclared within a
function definition; such a class is called a local class. The
name of a local class is local to its enclosing scope. The local
class is in the scope of the enclosing scope, and has the same access
to names outside the function as does the enclosing function.
Declarations in a local class can use only type names, static
variables, extern variables and functions, and enumerators
from the enclosing scope.

... The class-name in a base-specifier shall not be an
incompletely defined class (clause 9
class);
this class is called a
directbaseclass for the class being
declareddefined. During the lookup for a
base class name, non-type names are ignored
(3.3.7
basic.scope.hiding).
If the name found is not a class-name, the program is ill-formed.
A class B is a base class of a class D if it is a
direct base class of D or a direct base class of one of
D's base classes. A class is an indirect base class
of another if it is a base class but not a direct base class. A class
is said to be (directly or indirectly) derived from its (direct
or indirect) base classes. [Note: See clause
11
class.access
for the meaning of access-specifier.] Unless
redefinedredeclared in the
derived class, members of a base class are also considered to be
members of the derived class. The base class members are said to be
inherited by the derived class. Inherited members can be
referred to in expressions in the same manner as other members of the
derived class, unless their names are hidden or ambiguous
(10.2
class.member.lookup).
[Note: the scope resolution operator ::
(5.1
expr.prim)
can be used to refer to a direct or indirect base member explicitly.
This allows access to a name that has been redefinedredeclared in the derived class. A derived class can itself
serve as a base class subject to access control; see
11.2
class.access.base.
A pointer to a derived class can be implicitly converted to a pointer
to an accessible unambiguous base class
(4.10
conv.ptr).
An lvalue of a derived class type can be bound to a reference to an
accessible unambiguous base class
(8.5.3
dcl.init.ref).]

[Footnote: As specified previously in clause
11
class.access, private members of
a base class remain inaccessible even to derived classes unless
friend declarations within the base class
declarationdefinition are used to grant access
explicitly.]

Rationale: Storage class specifiers don't have any meaning when
associated with a type. In C++, class members can be
defineddeclared with the static
storage class specifier. Allowing storage class specifiers on type
declarations could render the code confusing for users.

383.
Is a class with a declared but not defined destructor a POD?

The standard (9
class par. 4) says that
"A POD-struct is an aggregate class
that has no non-static data members of type non-POD-struct,
non-POD-union (or array of such types) or reference, and has no
user-defined copy assignment operator and no user-defined destructor."

Note that it says 'user-defined', not 'user-declared'. Is it the
intent that if e.g. a copy assignment operator is declared but not
defined, this does not (per se) prevent the class to be a POD-struct?

A POD-struct is an aggregate class that has no non-static data
members of type non-POD-struct, non-POD-union (or array of such types)
or reference, and has no user-defineddeclared
copy assignment operator and no user-defineddeclared destructor. Similarly, a POD-union is an
aggregate union that has no non-static data members of type
non-POD-struct, non-POD-union (or array of such types) or reference,
and has no user-defineddeclared copy
assignment operator and no user-defineddeclared destructor.

Drafting note: The changes are shown relative to TC1, incorporating
the changes from the resolution of
core issue 148.

413.
Definition of "empty class"

The proposal says that value is true if "T is an empty class (10)".
Clause 10 doesn't define an empty class, although it has a note that
says a base class may "be of zero size (clause 9)" 9/3 says "Complete
objects and member subobjects of class type shall have nonzero size."
This has a footnote, which says "Base class subobjects are not so
constrained."

The standard uses the term "empty class" in two places
(8.5.1
dcl.init.aggr), but neither of
those places defines it. It's also listed in the index, which refers to
the page that opens clause 9, i.e. the nonzero size stuff cited above.

So, what's the definition of "empty class" that determines whether the
predicate is_empty is true?

The one place where it's used is
8.5.1
dcl.init.aggr paragraph 8, which says (roughly paraphrased)
that an aggregate initializer for an empty class must be "{}", and when
such an initializer is used for an aggregate that is not an empty class the
members are default-initialized. In this context it's pretty clear what's
meant. In the type traits proposal it's not as clear, and it was probably
intended to have a different meaning. The boost implementation, after it
eliminates non-class types, determines whether the trait is true by
comparing the size of a class derived from T to the size of an
otherwise-identical class that is not derived from T.

Howard Hinnant:is_empty was created to find out whether a type could be derived from
and have the empty base class optimization successfully applied. It
was created in part to support compressed_pair which attempts to
optimize away the space for one of its members in an attempt to reduce
spatial overhead. An example use is:

Here the compare function is optimized away via the empty base
optimization if Compare turns out to be an "empty" class. If Compare
turns out to be a non-empty class, or a function pointer, the space is
not optimized away. is_empty is key to making this work.

Clark Nelson:
I've been looking at issue 413, and I've reached the conclusion that
there are two different kinds of empty class. A class containing only
one or more anonymous bit-field members is empty for purposes of
aggregate initialization, but not (necessarily) empty for purposes of
empty base-class optimization.

Of course we need to add a definition of emptiness for purposes of
aggregate initialization. Beyond that, there are a couple of questions:

Should the definition of emptiness used by the is_empty predicate be
defined in a language clause or a library clause?

Do we need to open a new core issue pointing out the fact that the
section on aggregate initialization does not currently say that unnamed
bit-fields are skipped?

Notes from the October, 2005 meeting:

There are only two places in the Standard where the phrase
“empty class” appears, both in 8.5.1
dcl.init.aggr paragraph 8. Because it is not clear whether the
definition of “empty for initialization purposes” is
suitable for use in defining the is_empty predicate, it would
be better just to avoid using the phrase in the language clauses. The
requirements of 8.5.1
dcl.init.aggr paragraph 8 appear to be
redundant; paragraph 6 says that an initializer-list must have
no more initializers than the number of elements to initialize, so an
empty class already requires an empty initializer-list, and
using an empty initializer-list with a non-empty class is
covered adequately by paragraph 7's description of the handling of an
initializer-list with fewer initializers than the
number of members to initialize.

An initializer for an aggregate member that is an empty
class shall have the form of an empty initializer-list{}. [Example:

struct S { };
struct A {
S s;
int i;
} a = { { } , 3 };

—end example] An empty initializer-list can be used to
initialize any aggregate. If the aggregate is not an empty class, then
each member of the aggregate shall be initialized with a value of the
form T() (5.2.3
expr.type.conv), where T
represents the type of the uninitialized member.

Deleting 8.5.1
dcl.init.aggr paragraph 8 altogether may
not be a good idea. It would appear that, in its absence, the
initializer elision rules of paragraph 11 would allow the initializer
for a in the preceding example to be written { 3 }
(because the empty-class member s would consume no
initializers from the list).

Proposed resolution (October, 2006):

(Drafting note: this resolution also cleans up incorrect
references to syntactic non-terminals in the nearby paragraphs.)

An array of unknown size initialized with a brace-enclosed
initializer-list containing ninitializersinitializer-clauses, where n shall be greater
than zero... An empty initializer list {} shall not be used as the
initializerinitializer-clause for an array of
unknown bound.

An initializer for an aggregate member that is an empty
class shall have the form of an empty initializer-list{}. [Example:

struct S { };
struct A {
S s;
int i;
} a = { { } , 3 };

—end example] An empty initializer-list can be used to
initialize any aggregate. If the aggregate is not an empty class, then
each member of the aggregate shall be initialized with a value of the
form T() (5.2.3
expr.type.conv), where T
represents the type of the uninitialized member.

with:

If an aggregate class C contains a subaggregate
member m that has no members for purposes of aggregate
initialization, the initializer-clause for m shall not
be omitted from an initializer-list for an object of type
C unless the initializer-clauses for all members
of C following m are also omitted. [Example:

Braces can be elided in an initializer-list as follows. If the
initializer-list begins with a left brace, then the succeeding
comma-separated list of initializersinitializer-clauses initializes the members of a
subaggregate; it is erroneous for there to be more initializersinitializer-clauses than members. If, however,
the initializer-list for a subaggregate does not begin with a
left brace, then only enough initializersinitializer-clauses from the list are taken to
initialize the members of the subaggregate; any
remaining initializersinitializer-clauses
are left to initialize the next member of the aggregate of which the
current subaggregate is a member. [Example:...

All implicit type conversions (clause 4
conv) are
considered when initializing the aggregate member with an initializer
from an initializer-listassignment-expression. If
the initializerassignment-expression can
initialize a member, the member is initialized. Otherwise, if the
member is itself a non-empty subaggregate, brace elision is
assumed and the initializerassignment-expression is considered for the
initialization of the first member of the
subaggregate. [Note: As specified above, brace elision
cannot apply to subaggregates with no members for purposes of
aggregate initialization; an initializer-clause for the entire
subobject is required. —end note]
[Example:... Braces are elided around
the initializerinitializer-clause for
b.a1.i...

There are several problems with the terms defined in
9
class paragraph 4:

A structure is a class defined with
the class-keystruct; its members and base classes
(clause 10
class.derived) are public by default (clause
11
class.access). A union is a class defined with the
class-keyunion; its members are public by default and
it holds only one data member at a time (9.5
class.union). [Note: aggregates of class type are described
in 8.5.1
dcl.init.aggr. —end note]
A POD-struct is an aggregate class that has no non-static data
members of type non-POD-struct, non-POD-union (or array of such types)
or reference, and has no user-declared copy assignment operator and no
user-declared destructor. Similarly, a POD-union is an
aggregate union that has no non-static data members of type
non-POD-struct, non-POD-union (or array of such types) or reference,
and has no user-declared copy assignment operator and no user-declared
destructor. A POD class is a class that is either a POD-struct
or a POD-union.

Although the term structure is defined here, it is
used only infrequently throughout the Standard, often apparently
inadvertently and consequently incorrectly:

5.2.5
expr.ref paragraph 4: the use is in a
note and is arguably correct and helpful.

9.2
class.mem paragraph 11: the term is used
(three times) in an example. There appears to be no reason to use
it instead of “class,” but its use is not
problematic.

17.1.8
defns.iostream.templates: the traits argument
to the iostream class templates is (presumably unintentionally)
constrained to be a structure, i.e., to use the struct
keyword and not the class keyword in its definition.

B
limits paragraph 2: the minimum number of
declarator operators is given for structures and unions but not
for classes defined using the class keyword.

B
limits paragraph 2: class, structure,
and union are used as disjoint terms in describing nesting
levels. (The nonexistent nonterminal struct-declaration-list
is used, as well.)

There does not appear to be a reason for defining the term
structure. The one reference where it is arguably useful,
in the note in 5.2.5
expr.ref, could be rewritten
as something like, “'class objects' may be defined using the
class, struct, or unionclass-keys; see clause 9
class.”

Based on its usage later in the paragraph and elsewhere,
“POD-struct” appears to be intended to exclude unions.
However, the definition of “aggregate class” in
8.5.1
dcl.init.aggr paragraph 1 includes unions.
Furthermore, the name itself is confusing, leading to the question of
whether it was intended that only classes defined
using struct could be POD-structs or
if class-classes are included. The definition should
probably be rewritten as, “A POD-struct is an aggregate
class defined with the class-keystruct or
the class-keyclass that has no...

In most references outside clause 9
class,
POD-struct and POD-union are mentioned together and treated
identically. These references should be changed to refer to the
unified term, “POD class.”

Noted in passing: 18.1
lib.support.types paragraph 4
refers to the undefined terms “POD structure” and
(unhyphenated) “POD union;” the pair should be replaced
by a single reference to “POD class.”

A structure is a class defined with
the class-keystruct; its members and base classes
(clause 10
class.derived) are public by default (clause
11
class.access). A union is a class defined with the
class-keyunion; its members are public by default
and it holds only one data member at a time (9.5
class.union). [Note: aggregates of class type are described
in 8.5.1
dcl.init.aggr. —end note]
A POD-struct is an aggregate class that has no non-static
data members of type non-POD-struct, non-POD-union (or array of such
types) or reference, and has no user-declared copy assignment operator
and no user-declared destructor. Similarly, a POD-union is an
aggregate union that has no non-static data members of type
non-POD-struct, non-POD-union (or array of such types) or reference,
and has no user-declared copy assignment operator and no user-declared
destructor. A POD class is a class that is either a POD-struct
or a POD-union.A POD class is an aggregate class that
has no non-static data members of non-POD type (or array of such a
type) or reference, and has no user-declared copy assignment operator
and no user-declared destructor. A POD-struct is a POD class
defined with the class-keystruct or
the class-keyclass. A POD-union is a POD class
defined with the class-keyunion.

In the absence of an access-specifier for a base
class, public is assumed when the derived class is
declareddefined with
the class-keystruct and private is
assumed when the class is declareddefined with the
class-keyclass. [Example:...

417.
Using derived-class qualified name in out-of-class nested class definition

We had a user complain that our compiler was allowing the
following code:

struct B {
struct S;
};
struct D : B { };
struct D::S {
};

We took one look at the code and made the reasonable (I would claim)
assumption that this was
indeed a bug in our compiler. Especially as we had just fixed a very
similar issue with the
definition of static data members.

Imagine our surprise when code like this showed up in Boost and that
every other compiler we tested accepts this code. So is this
indeed legal (it seems like
it must be) and if so is there any justification for this beyond
3.4.3.1
class.qual?

John Spicer:
The equivalent case for a member function is covered by the declarator
rules in 8.3
dcl.meaning paragraph 1. The committee has
previously run into cases where a restriction should apply to both
classes and non-classes, but fails to do so because there is no
equivalent of 8.3
dcl.meaning paragraph 1 for classes.

Given that, by the letter of the standard, I would say that this
case is allowed.

Add the following as a new paragraph immediately following
3.3.1
basic.scope.pdecl paragraph 2:

The point of declaration for a class first declared by a
class-specifier is
immediately after the identifier
or template-id (if any) in its class-head
(Clause 9
class).
The point of declaration for an enumeration is immediately after
the identifier (if any) in its enum-specifier
(7.2
dcl.enum).

The potential scope of a name declared in a class consists not only of the
declarative region following the name's
declaratorpoint of declaration, but also
of all function bodies, default arguments, and constructor
ctor-initializers in that
class (including such things in nested classes).

[Note that the preceding change duplicates one of the changes
in the proposed resolution of issue 432.]

If the explicit instantiation is for a member function, a member
class or a static
data member of a class template specialization, the name of
the class template
specialization in the qualified-id for the member declaratorname shall be a template-id.

If a class-head contains a nested-name-specifier,
the class-specifier shall refer to a
class that was previously declared directly in the class or namespace
to which the nested-name-specifier refers (i.e., neither
inherited nor introduced by a using-declaration),
and the class-specifier shall appear in a namespace enclosing the
previous declaration.

When a nested-name-specifier is specified in a
class-head or in an
elaborated-type-specifier, the resulting qualified name
shall refer to a previously declared member of the class or namespace to
which the nested-name-specifier refers, and
the member shall not have been introduced by a
using-declaration in the scope of the class or namespace
nominated by the nested-name-specifier.

328.
Missing requirement that class member types be complete

Is it legal to use an incomplete type (3.9
basic.types
paragraph 6) as a
class member, if no object of such class is ever created ?

And as a class template member, even if the template is instantiated,
but no object of the instantiated class is created?

The consensus seems to be NO, but no wording was found in the standard
which explicitly disallows it.

The problem seems to be that most of the restrictions on incomplete types
are on their use in objects, but class members are not objects.

A possible resolution, if this is considered a defect, is to add to
3.2
basic.def.odr paragraph 4, (situations when T must be complete),
the use of T as a member of a class or instantiated class template.

The thread on comp.std.c++ which brought up the issue was
"Compiler differences: which is correct?", started 2001 11 30.
<3c07c8fb$0$8507$ed9e5944@reading.news.pipex.net>

Proposed Resolution (April 2002, revised April 2003):

Change the first bullet of the note in 3.2
basic.def.odr
paragraph 4 and add two new bullets following it, as follows:

Non-static (9.4
class.static) data members shall
not have incomplete types.
In particular, a class C shall not contain a non-static
member of class C, but it can contain a pointer or reference to
an object of class C.

See also 3.9
basic.types paragraph 6, which is relevant
but not changed by the Proposed Resolution.

437.
Is type of class allowed in member function exception specification?

The compiler is complaining that Foo is an incomplete type, and can't be
used in the exception specification.

My reading of the standard [basic.types] is inconclusive. Although it
does state that the class declaration is considered complete when the
closing brace is read, I believe it also intends that the member
function declarations should not be semantically validated until the
class has been completely declared.

If this isn't allowed, I don't know how else a member function could be
declared to throw an object of its own class.

John Spicer:
The type is considered complete within function bodies, but not in their
declaration (see 9.2
class.mem paragraph 2).

Within the class member-specification, the class is regarded
as complete within function bodies, default arguments,
exception-specifications, and constructor
ctor-initializers (including such things in nested classes).

Rationale: Taken with 8.3.5
dcl.fct paragraph 6, the
exception-specification is the only part of a function
declaration/definition in which the class name cannot be used because
of its putative incompleteness. There is no justification for
singling out exception specifications this way; both in the function
body and in a catch clause, the class type will be complete,
so there is no harm in allowing the class name to be used in the
exception-specification.

613.
Unevaluated uses of non-static class members

According to 9.2
class.mem paragraph 9, the name of a
non-static data member can only be used with an object reference
(explicit or implied by the this pointer of a non-static
member function) or to form a pointer to member. This restriction
applies even in the operand of sizeof, although the
operand is not evaluated and thus no object is needed to perform the
operation. Consequently, determining the size of a non-static class
member often requires a circumlocution like

sizeof((C*) 0)->m

instead of the simpler and more obvious (but incorrect)

sizeof(C::m)

The CWG considered this question as part of
issue 198 and decided at that time to
retain the restriction on consistency grounds: the rule was
viewed as applying uniformly to expressions, and making an
exception for sizeof would require introducing a
special-purpose “wart.”

The issue has recently resurfaced, in part due to the fact that
the restriction would also apply to the decltype operator.
Like the unary & operator to form a pointer to member,
sizeof and decltype need neither an lvalue nor
an rvalue, requiring solely the declarative information of the
named operand. One possible approach would be to define the concept
of “unevaluated operand” or the like, exempt unevaluated
operands from the requirement for an object reference in
9.2
class.mem paragraph 9, and then define the operands
of these operators as “unevaluated.”

We are not quite sure about what an "unnamed class" is.
There is no exact definition in ISO/IEC 14882; the closest we can come to a
hint is the wording of section 7.1.3
dcl.typedef
paragraph 5, where it seems to be understood that a class-specifier with no
identifier between "class" and "{" is unnamed. The identifier provided after
"}" ( "A" in the test case above) is there for "linkage purposes" only.

To us, class B::A in the test program above seems "named"
enough, and there is certainly a mechanism to provide the definition for
B::A::i (in contrast to the note in section 9.4.2
class.static.data
paragraph 5).

Our position is therefore that the above test program is
indeed legal C++. Can you confirm or reject this claim?

Herb Sutter replied to the submitter as follows:
Here are my notes based on a grep for
"unnamed class" in the standard:

3.5
basic.link paragraph 4, bullet 3, makes a note that
does not directly speak to your question but which draws the same distinction:

a named class (clause class), or an unnamed class defined in a
typedef declaration in which the class has the typedef name for
linkage purposes (7.1.3
dcl.typedef);

Likewise in your example, you have an unnamed class defined in a
typedef declaration.

9.4.2
class.static.data paragraph 5 does indeed appear to
me to make your example not supported by ISO C++ (although implementations
could allow it as an extension, and many implementation do happen to allow it).

7.1.3
dcl.typedef paragraph 5 does indeed likewise confirm
the interpretation you give of an unnamed class.

So yes, an unnamed class is one where there is no identifier (class
name) between the class-key and the {. This is also in harmony with the
production for class-name in 9
class paragraph 1:

class-name:

identifiertemplate-id

Notes from the October 2003 meeting:

We agree that the example is not valid; this is an unnamed class.
We will add wording to define an unnamed class. The note in
9.4.2
class.static.data paragraph 5 should be corrected
or deleted.

454.
When is a definition of a static data member required?

As a result of the resolution of core issue 48, the current C++ standard is not in sync with existing
practice and with user expectations as far as definitions of static
data members having const integral or const enumeration type are
concerned. Basically what current implementations do is to require a
definition only if the address of the constant is taken. Example:

To the letter of the standard, though, the above requires a
definition of npos, since the
expression std::string::npos is potentially evaluated. I
think this problem would be easily solved with simple changes to
9.4.2
class.static.data paragraph 4, 9.4.2
class.static.data paragraph 5 and 3.2
basic.def.odr paragraph
3.

If a static data member is of const integral or const enumeration
type, its declaration in the class definition can specify a
constant-initializer which shall be [note1] an integral constant
expression (5.19). In that case, the member can appear in integral
constant expressions. No definition of the member is required, unless
an lvalue expression that designates it is potentially evaluated and
either used as operand to the built-in unary & operator [note 2] or
directly bound to a reference.

If a definition exists, it shall be at namespace scope and shall not
contain an initializer.

More information, in response to a question about why
issue 48 does not resolve the problem:

The problem is that the issue was settled in a way that solves much
less than it was supposed to solve; that's why I decided to file, so
to speak, a DR on a DR.

I understand this may seem a little 'audacious' on my part, but please
keep reading. Quoting from the text of DR 48 (emphasis mine):

Originally, all static data members still had to be defined
outside the class whether they were used or not.

But that restriction was supposed to be lifted [...]

In particular, if an integral/enum const static data member is
initialized within the class, and its address is never taken,
we agreed that no namespace-scope definition was required.

The corresponding resolution doesn't reflect this intent, with the
definition being still required in most situations anyway: it's enough
that the constant appears outside a place where constants are
required (ignoring the obvious cases of sizeof and typeid) and you
have to provide a definition. For instance:

whose main goal is to map the same couples (type, value) to the same
storage, also solves the definition problem. In this usage it is an
excellent hack (if your compiler is good enough), but IMHO still a
hack on a language defect.

<end digression>

What I propose is to solve the issue according to the original intent,
which is also what users expect and all compilers that I know of
already do. Or, in practice, we would have a rule that exists only as
words in a standard document.

PS: I've sent a copy of this to Mr. Adamczyk to clarify an important
doubt that occurred to me while writing this reply:

if no definition is provided for an integral static const data member
is that member an object? Paragraph 1.8/1 seems to say no, and in fact
it's difficult to think it is an object without assuming/pretending
that a region of storage exists for it (an object *is* a region of
storage according to the standard).

I would think that when no definition is required we have to assume
that it could be a non-object. In that case there's nothing in 3.2
which says what 'used' means for such an entity and the current
wording would thus be defective. Also, since the name of the member is
an lvalue and 3.10/2 says an lvalue refers to an object we would have
another problem.

OTOH the standard could pretend it is always an object (though the
compiler can optimize it away) and in this case it should probably
make a special case for it in 3.2/2.

Notes from the March 2004 meeting:

We sort of like this proposal, but we don't feel it has very
high priority. We're not going to spend time discussing it, but
if we get drafting for wording we'll review it.

An expression is potentially evaluated unless it appears where
an integral constant expression is required (see 5.19
expr.const), is the operand of the sizeof operator
(5.3.3
expr.sizeof), or is the operand of
the typeid operator and the expression does not designate an
lvalue of polymorphic class type (5.2.8
expr.typeid). An
object or non-overloaded function is used if its name appears
in a potentially-evaluated expression.

to

An expression that is the operand of the sizeof operator
(5.3.3
expr.sizeof) is unevaluated, as is an
expression that is the operand of the typeid operator if it
is not an lvalue of a polymorphic class type (5.2.8
expr.typeid); all other expressions are potentially
evaluated. An object or non-overloaded function whose name appears
as a potentially-evaluated expression is used, unless it is an
object that satisfies the requirements for appearing in an integral
constant expression (5.19
expr.const) and the lvalue-to-rvalue
conversion (4.1
conv.lval) is immediately applied.

If a static data member is of const integral or const enumeration
type, its declaration in the class definition can specify a
constant-initializerwhichwhose constant-expression
shall be an integral constant expression (5.19
expr.const).

58.
Signedness of bit fields of enum type

Section 9.6
class.bit
paragraph 4 needs to be more specific about the signedness of bit
fields of enum type. How much leeway does an implementation have
in choosing the signedness of a bit field? In particular, does the
phrase "large enough to hold all the values of the enumeration" mean "the
implementation decides on the signedness, and then we see whether all the
values will fit in the bit field", or does it require the implementation
to make the bit field signed or unsigned if that's what it takes to make
it "large enough"?

Note (March, 2005): Clark Nelson observed that there is
variation among implementations on this point.

Notes from April, 2005 meeting:

Although implementations enjoy a great deal of latitude in handling
bit-fields, it was deemed more user-friendly to ensure that the
example in paragraph 4 will work by requiring implementations to use
an unsigned underlying type if the enumeration type has no negative
values. (If the implementation is allowed to choose a signed
representation for such bit-fields, the comparison against
TRUE will be false.)

In addition, it was observed that there is an apparent circularity
between 7.2
dcl.enum paragraph 7 and 9.6
class.bit paragraph 4 that should be resolved.

For an enumeration where emin is the smallest
enumerator and emax is the largest, the values of
the enumeration are the values in the range bmin to
bmax, defined as follows: Let K be 1 for a
two's complement representation and 0 for a one's complement or
sign-magnitude representation. bmax is the smallest
value greater than or equal to
max(|emin|-K,|emax|) and
equal to 2M-1, where M is a non-negative
integer. bmin is zero if emin is
non-negative and -(bmax+K) otherwise. The
size of the smallest bit-field large enough to hold all the values of
the enumeration type is max(M,1) if bmin is
zero and M+1 otherwise. It is possible to define an
enumeration that has values not defined by any of its enumerators.

Add the indicated text to the second sentence of
9.6
class.bit paragraph 4:

If the value of an enumerator is stored into a bit-field of the same
enumeration type and the number of bits in the bit-field is large
enough to hold all the values of that enumeration type
(7.2
dcl.enum), the original enumerator value and
the value of the bit-field shall compare equal.

198.
Definition of "use" in local and nested classes

Declarations in a local class can use only type names, static
variables, extern variables and functions, and enumerators
from the enclosing scope.

The definition of when an object or function is "used" is found in
3.2
basic.def.odr
paragraph 2 and
essentially says that the operands of sizeof and
non-polymorphic typeid operators are not used. (The
resolution for issue 48 will add
contexts in which integral constant expressions are required to the
list of non-uses.)

This definition of "use" would presumably allow code like

void foo() {
int i;
struct S {
int a[sizeof(i)];
};
};

which is required for C compatibility.

However, the restrictions on nested classes in
9.7
class.nest
paragraph 1 are very
similar to those for local classes, and the example there explicitly
states that a reference in a sizeof expression is a forbidden
use (abbreviated for exposition):

[As a personal note, I have seen real-world code that was exactly like
this; it was hard to persuade the author that the required
writearound, sizeof(((enclose*) 0)->x), was an improvement
over sizeof(x). —wmm]

Similarly, 9.2
class.mem paragraph 9 would appear to
prohibit examples like the following:

struct B {
char x[10];
};
struct D: B {
char y[sizeof(x)];
};

Suggested resolution: Add cross-references to
3.2
basic.def.odr
following the word
"use" in both 9.7
class.nest
and
9.8
class.local
, and change the example
in 9.7
class.nest
to indicate that a
reference in a sizeof expression is permitted. In
9.2
class.mem paragraph 9, "referred to" should be
changed to "used" with a cross_reference to
3.2
basic.def.odr.

Notes from 10/01 meeting:

It was noted that the suggested resolution did not make the
sizeof() example in 9.7
class.nest valid.
Although the reference to the argument of sizeof() is not regarded
as a use, the right syntax must be used nonetheless to reference a non-static
member from the enclosing class. The use of the member name by itself is not
valid. The consensus within the core working group was that nothing should be
done about this case. It was later discovered that
9.4
class.static paragraph 3 states that

The
definition of a static
member shall not use directly the names of the nonstatic members of its class
or of a base class of its class (including as operands of the sizeof
operator). The definition of a static member may only refer to
these members to
form pointer to members (5.3.1
expr.unary.op) or
with the class member access syntax (5.2.5
expr.ref).

This seems to
reinforce the decision of the working group.

The use of "use" should still be cross-referenced. The
statements in 9.7
class.nest and 9.8
class.local
should also be rewritten to state the requirement
positively rather than negatively as the list of "can't"s is already missing
some cases such as template parameters.

Notes from the 4/02 meeting:

We backed away from "use" in the technical sense, because the
requirements on the form of reference are the same whether or not
the reference occurs inside a sizeof.

Except when used to form a pointer to member
(5.3.1
expr.unary.op), when used in the body of a
nonstatic member function of
its class or of a class derived from its class
(9.3.1
class.mfct.nonstatic), or when used in
a mem-initializer for a
constructor for its class or for a class derived from its class
(12.6.2
class.base.init), a nonstatic data or function member
of a class shall only be referred to with the class member access syntax
(5.2.5
expr.ref).

with the following paragraph

Each occurrence in an expression of the name of a nonstatic data
member or nonstatic member function of a class shall be expressed as a class
member access (5.2.5
expr.ref), except when it appears
in the formation of a pointer to member
(5.3.1
expr.unary.op), when it appears in the body
of a nonstatic member function
of its class or of a class derived from its class
(9.3.1
class.mfct.nonstatic), or when it appears in a
mem-initializer for
a constructor for its class or for a class derived from its class
(12.6.2
class.base.init).

Except by using explicit pointers, references, and object names,
declarations in a nested class can use only type names, static members, and
enumerators from the enclosing class.

with the following

[Note: In accordance with 9.2
class.mem,
except by using explicit pointers,
references, and object names, declarations in a nested class shall not use
nonstatic data members or nonstatic member functions from the
enclosing class. This restriction applies in all constructs including the
operands of the sizeof operator.]

In the example following 9.7
class.nest
paragraph 1, change the comment on the first statement of
function f to emphasize that sizeof(x) is an error. The
example reads in full:

484.
Can a base-specifier name a cv-qualified class type?

Issue 298, recently approved,
affirms that cv-qualified class types can be used as
nested-name-specifiers. Should the same be true for
base-specifiers?

Rationale (April, 2005):

The resolution of issue 298 added
new text to 9.1
class.name paragraph 5 making it
clear that a typedef that names a cv-qualified class type is a
class-name. Because the definition of base-specifier
simply refers to class-name, it is already the case that
cv-qualified class types are permitted as base-specifiers.

Additional notes (June, 2005):

It's not completely clear what it means to have a cv-qualified
type as a base-specifier. The original proposed resolution
for issue 298 said that “the
cv-qualifiers are ignored,” but that wording is not in the
resolution that was ultimately approved.

If the cv-qualifiers are not ignored, does that mean that
the base-class subobject should be treated as always similarly
cv-qualified, regardless of the cv-qualification of the derived-class
lvalue used to access the base-class subobject? For instance:

A typedef-name (7.1.3
dcl.typedef) that names a
class type, or a cv-qualified version thereof, is also
a class-name, butclass-name. If
a typedef-name that names a cv-qualified class type is used
where a class-name is required, the cv-qualifiers are ignored.
A typedef-name shall not be used as the identifier
in a
class-head.

... Each of these declarations that was introduced by a using-declaration
is considered to be from each sub-object of C that is of the type containing
the declaration designated by the using-declaration. If the resulting set
of declarations are not all from sub-objects of the same type, or the set
has a nonstatic member and includes members from distinct sub-objects,
there is an ambiguity and the program is ill-formed.

This contradicts the text and example in paragraph 12 of 7.3.3
namespace.udecl
.

The resulting set of declarations shall all be from sub-objects
of the same type, or there shall be a set of declarations from
sub-objects of a single type that contains using-declarations
for the declarations found in all other sub-object types.
Furthermore, for nonstatic members, the resulting set of
declarations shall all be from a single sub-object, or there shall
be a set of declarations from a single sub-object that contains
using-declarations for the declarations found in all other
sub-objects. Otherwise, there is an ambiguity and the program is
ill-formed.

... Each of these declarations that was introduced by a using-declaration
is considered to be from each sub-object of C that is of the type containing
the declaration designated by the using-declaration. If the resulting set
of declarations are not all from sub-objects of the same type, or the set
has a nonstatic member and includes members from distinct sub-objects,
there is an ambiguity and the program is ill-formed.

Since the two x's are considered to be "from" different objects, looking up
x produces a set including declarations "from" different objects, and the
program is ill-formed. Clearly this is wrong. The problem with the
existing wording is that it fails to consider lookup context.

The first proposed solution:

The resulting set of declarations shall all be from sub-objects
of the same type, or there shall be a set of declarations from
sub-objects of a single type that contains using-declarations
for the declarations found in all other sub-object types.
Furthermore, for nonstatic members, the resulting set of
declarations shall all be from a single sub-object, or there shall
be a set of declarations from a single sub-object that contains
using-declarations for the declarations found in all other
sub-objects. Otherwise, there is an ambiguity and the program is
ill-formed.

because it considers the lookup context, but not the definition context;
under this definition of "from", the two declarations found are the
using-declarations, which are "from" B1 and B2.

The solution is to separate the notions of lookup and definition context.
I have taken an algorithmic approach to describing the strategy.

Incidentally, the earlier proposal allows one base to have a superset of
the declarations in another base; that was an extension, and my proposal
does not do that. One algorithmic benefit of this limitation is to
simplify the case of a virtual base being hidden along one arm and not
another ("domination"); if we allowed supersets, we would need to remember
which subobjects had which declarations, while under the following
resolution we need only keep two lists, of subobjects and declarations.

The following steps define the result of name lookup for a member
name f in a class scope C.

The lookup set for f in C, called S(f,C), consists of two
component sets: the declaration set, a set of members named f; and
the subobject set, a set of subobjects where declarations of these
members (possibly including using-declarations) were found. In the
declaration set, using-declarations are replaced by the members they
designate, and type declarations (including injected-class-names) are
replaced by the types they designate. S(f,C) is calculated as
follows.

If C contains a declaration of the name f, the declaration set contains
every declaration of f in C (excluding bases), the subobject set contains C
itself, and calculation is complete.

Otherwise, S(f,C) is initially empty. If C has base classes, calculate
the lookup set for f in each direct base class subjobject Bi,
and merge each such lookup set S(f,Bi) in turn into S(f,C).

The following steps define the result of merging lookup set
S(f,Bi) into the intermediate S(f,C):

If each of the subobject members of S(f,Bi) is a base
class subobject of at least one of the subobject members of S(f,C), S(f,C)
is unchanged and the merge is complete. Conversely, if each of the
subobject members of S(f,C) is a base class subobject of at least one of
the subobject members of S(f,Bi), the new S(f,C) is a copy of
S(f,Bi).

Otherwise, if the declaration sets of S(f,Bi) and S(f,C)
differ, the merge is ambiguous: the new S(f,C) is a lookup set with an
invalid declaration set and the union of the subobject sets. In subsequent
merges, an invalid declaration set is considered different from any
other.

Otherwise, consider each declaration d in the set, where d is a
member of class A. If d is a nonstatic member, compare the A base class
subobjects of the subobject members of S(f,Bi) and
S(f,C). If they do not match, the merge is ambiguous, as in the
previous step. [Note: It is not necessary to remember which A subobject
each member comes from, since using-declarations don't disambiguate. ]

Otherwise, the new S(f,C) is a lookup set with the shared
set of declarations and the union of the subobject sets.

The result of name lookup for f in C is the declaration set of
S(f,C). If it is an invalid set, the program is ill-formed.

Mike Miller raised some new issues in N1543, and we adjusted the
proposed resolution as indicated in that paper.

Further information from Mike Miller (January 2004):

Unfortunately, I've become aware of a minor glitch in
the proposed resolution for issue 39 in N1543, so I'd like to
suggest a change that we can discuss in Sydney.

A brief review and background of the problem: the major change we
agreed on in Kona was to remove detection of multiple-subobject
ambiguity from class lookup (10.2
class.member.lookup) and instead
handle it as part
of the class member access expression. It was pointed out in
Kona that 11.2
class.access.base/5 has this effect:

If a class member access operator, including an implicit
"this->," is used to access a nonstatic data member or
nonstatic member function, the reference is ill-formed if the
left operand (considered as a pointer in the "." operator
case) cannot be implicitly converted to a pointer to the
naming class of the right operand.

After the meeting, however, I realized that this requirement is
not sufficient to handle all the cases. Consider, for instance,

Here, both the object expression ("this") and the naming class
are "D", so the reference to "i" satisfies the requirement in
11.2
class.access.base/5, even though it involves a
multiple-subobject ambiguity.

In order to address this problem, I proposed in N1543 to add a
paragraph following 5.2.5
expr.ref/4:

If E2 is a non-static data member or a non-static member
function, the program is ill-formed if the class of E1 cannot
be unambiguously converted (10.2) to the class of which E2 is
directly a member.

That's not quite right. It does diagnose the case above as
written; however, it breaks the case where qualification is used
to circumvent the ambiguity:

In my proposed wording, the class of "this" can't be converted to
"B" (the qualifier is ignored), so the access is ill-formed.
Oops.

I think the following is a correct formulation, so the proposed
resolution we discuss in Sydney should contain the following
paragraph instead of the one in N1543:

If E2 is a nonstatic data member or a non-static member
function, the program is ill-formed if the naming class
(11.2) of E2 cannot be unambiguously converted (10.2) to
the class of which E2 is directly a member.

This reformulation also has the advantage of pointing readers to
11.2
class.access.base, where the the convertibility requirement
from the class of
E1 to the naming class is located and which might otherwise be
overlooked.

Notes from the March 2004 meeting:

We discussed this further and agreed with these latest
recommendations. Mike Miller has produced a paper N1626 that gives
just the final collected set of changes.

390.
Pure virtual must be defined when implicitly called

A pure virtual function need be defined only if explicitly called with
the qualified-id syntax (5.1
expr.prim).

This is IMHO incomplete. A dtor is a function (well, a "special member
function", but this also makes it a function, right?) but it is called
implicitly and thus without a qualified-id syntax. Another alternative
is that the pure virtual function is called directly or indirectly from
the ctor. Thus the above sentence which specifies when a pure virtual
function need be defined ("...only if...") needs to be extended:

A pure virtual function need be defined only if explicitly called with
the qualified-id syntax (5.1
expr.prim) or if implicitly
called (12.4
class.dtor or 12.7
class.cdtor).

All access controls in clause 11 affect the ability to access a class
member name from a particular scope... In particular, access controls
apply as usual to member names accessed as part of a function return type,
even though it is not possible to determine the access privileges of that
use without first parsing the rest of the function declarator.

This means, if we take the loosest possible definition of "access from
a particular scope", that we have to save and check later the following
names

I suspect that member templates were not really considered when this was
written, and that it might have been written rather differently if they
had been. Note that access to the template arguments is only legal because
the class has been declared a friend, which is probably not what most programmers
would expect.

Rationale:

Not a defect. This behavior is as intended.

ISSUE 2:

Now consider void A::int_temp<A::A1, A::garbconst + 2, &A::func>::func1()
{...} By my reading of 11.8
class.access.nest
, the references to
A::A1, A::garbconst and A::func are now illegal,
and there is no way to define this function outside of the class. Is there
any need to do anything about either of these Issues?

Proposed resolution (04/01):

The resolution for this issue is contained in the resolution
for issue 45.

494.
Problems with the resolution of issue 45

The proposed resolution for issue 45 inserts the following sentence after 11
class.access paragraph 1:

A member of a class can also access all names as the class of
which it is a member.

I don't think that this is correctly constructed English. I see two
possibilities:

This is a typo, and the correct change is:

A member of a class can also access all names of the class of which
it is a member.

The intent is something more like:

A member of a nested class can also access all names accessible by
any other member of the class of which it is a member.

[Note: this was editorially corrected at the time defect
resolutions were being incorporated into the Working Paper to
read, “...can also access all the names declared in the
class of which it is a member,” which is essentially the
same as the preceding option 1.]

[Note: this means that access to private and protected
names is also granted to member functions of the friend class (as
if the functions were each friends) and to the static data member
definitions of the friend class. This also means that private and
protected type names from the class granting friendship can be
used in the base-clause of a nested class of the friend
class. However, the declarations of members of classes nested
within the friend class cannot access the names of private and
protected members from the class granting friendship. Also,
because the base-clause of the friend class is not part of
its member declarations, the base-clause of the friend
class cannot access the names of the private and protected
members from the class granting friendship. For example,

This seems to be an oversight. The proposed change to
11.8
class.access.nest paragraph 1 would appear to have
eliminated the restrictions on nested class access. However, at
least one compiler (gcc 3.4.3) doesn't appear to take my view,
and continues with the restrictions on access by classes within a
friend class, while implementing the rest of the resolution of
issue 45.

Like a member function, a friend function (11.4
class.friend)
defined within a nested class is in the lexical scope of that class;
it obeys the same rules for name binding as a static member function
of that class (9.4
class.static) and has no special access
rights to members of an enclosing class.

9.
Clarification of access to base class members

A base class is said to be accessible if an invented public
member of the base class is accessible. If a base class is accessible,
one can implicitly convert a pointer to a derived class to a pointer to
that base class.

Can you access the protected member b1 of B in foo?
Can you convert a D* to a B* in foo?

1st interpretation:

A public member of B is accessible within foo (since
foo is a friend), therefore foo can refer to b1 and convert
a D* to a B*.

2nd interpretation:

B is a protected base class of D, and a public member of B
is a protected member of D and can only be accessed within members
of D and friends of D. Therefore foo cannot refer to
b1 and cannot convert a D* to a B*.

A friend of a class is a function or class that is not a
member of the class but is allowedgiven permission
to use the private and protected member names from the class.
The name of a friend is not in the scope of the class, and
the friend is not called with the member access operators
(5.2.5
expr.ref) unless it is a member of
another class.A class specifies its friends, if any,
by way of friend declarations. Such declarations give special
access rights to the friends, but they do not make the nominated
friends members of the befriending class.

#410-D: protected function "A::g" is not accessible through a "B" pointer or object
b.g();
^

Steve Adamczyk:
The error in this case is due to
11.5
class.protected of the standard, which is an additional
check on top of the other access checking. When that section says
"a protected nonstatic member function ... of a base class" it doesn't
indicate whether the fact that there is a using-declaration is relevant.
I'd say the current wording taken at face value would suggest that
the error is correct -- the function is protected, even if the
using-declaration for it makes it accessible as a public function.
But I'm quite sure the wording in
11.5
class.protected was written before using-declarations
were invented and has not been reviewed since for consistency with
that addition.

When a friend or a member function of a derived class references a
protected nonstatic member function or protected nonstatic data member
of a base class, an access check applies in addition to those
described earlier in clause 11
class.access.
[Footnote: This
additional check does not apply to other members, e.g. static
data members or enumerator member constants.] Except
when forming a pointer to member (5.3.1
expr.unary.op),
the access must be through a
pointer to, reference to, or object of the derived class itself (or
any class derived from that class (5.2.5
expr.ref).
If the access is to form a
pointer to member, the nested-name-specifier shall name the
derived class (or any class derived from that class).

to

An additional access check beyond those described earlier in
clause 11
class.access
is applied when a nonstatic data member or nonstatic member
function is a protected member of its naming class
(11.2
class.access.base).
[Footnote: This additional check does not apply
to other members, e.g., static data members or enumerator member
constants.] As described
earlier, access to a protected member is granted because the
reference occurs in a friend or member of some
class C. If the access is to form a pointer to member
(5.3.1
expr.unary.op), the
nested-name-specifier shall name C or a
class derived from C. All other accesses involve a
(possibly implicit) object expression (5.2.5
expr.ref).
In this case, the
class of the object expression shall be C or a class derived
from C.

I think the formulation that the member is a protected member
of its naming class is not what we want. I think we intended
that the member is protected in the declaration that is found,
where the declaration found might be a using-declaration.

Mike Miller: I think the proposed wording makes the
access pb->p ill-formed, and I think that's the right
thing to do.

First, protected inheritance of A by B means
that B intends the public and protected members of
A to be part of B's implementation, available
to B's descendants only. (That's why there's a
restriction on converting from B* to A*, to
enforce B's intention on the use of members of
A.) Consequently, I see no difference in access policy
between your example and

class B {
protected:
int p;
};

Second, the reason we have this rule is that C's use
of inherited protected members might be different from their use
in a sibling class, say D. Thus members and friends of
C can only use B::p in a manner consistent with
C's usage, i.e., in C or
derived-from-C objects. If we rewrote your example
slightly,

This is because the nested class does not have access to the member E
in D. 11
class.access
paragraph 5 says that access to D::E is checked with
member access to class E, but unfortunately that doesn't give
access to D::E. 11
class.access
paragraph 6 covers the access for D::E::m,
but it doesn't affect the D::E access. Are there any implementations
that are standard compliant that support this?

Here is another example:

class C {
class B
{
C::B *t; //2 error, C::B is inaccessible
};
};

This causes trouble for member functions declared outside of the class
member list. For example:

If the return type (i.e. C::B) is access checked in the scope
of class B (as implied by
11
class.access
paragraph 5)
as a qualified name, then
the return type is an error just like referring to C::B in the
member list of class B above (i.e. //2) is ill-formed.

Proposed resolution (04/01):

The resolution for this issue is incorporated into the
resolution for issue 45.

The members of a nested class
have no special access to members of an enclosing class, nor to classes
or functions that have granted friendship to an enclosing class; the usual
access rules (clause 11
class.access
)
shall be obeyed. [...]

This makes me think that the ': Parent' part is OK by itself,
but that the implicit call of 'Parent::Parent()' by 'Derived::Derived()'
is not.

From Mike Miller:

I think it is completely legal, by the reasoning given in the (non-normative)
11.8
class.access.nest
paragraph 2. The use of a private nested class as a base of another nested class
is explicitly declared to be acceptable there. I think the rationale in
the comments in the example ("// OK because of injection of name A in A")
presupposes that public members of the base class will be public members
in a (publicly-derived) derived class, regardless of the access of the
base class, so the constructor invocation should be okay as well.

Insert the following as a new paragraph following
11
class.access paragraph 1:

A member of a class can also access all names as the class of which
it is a member. A local class of a member function may access the
same names that the member function itself may access. [Footnote:
Access permissions are thus transitive and cumulative to nested and
local classes.]

The members of a nested class have no special access to members of an
enclosing class, nor to classes or functions that have granted
friendship to an enclosing class; the usual access rules
(clause 11
class.access) shall be obeyed.

to

A nested class is a member and as such has the same access rights
as any other member.

263.
Can a constructor be declared a friend?

According to 12.1
class.ctor paragraph 1, a
declaration of a constructor has a special limited syntax, in
which only function-specifiers are allowed. A friend
specifier is not a function-specifier, so one interpretation
is that a constructor cannot be declared in a friend
declaration.

(It should also be noted, however, that neither friend
nor function-specifier is part of the declarator
syntax, so it's not clear that anything conclusive can be derived
from the wording of 12.1
class.ctor.)

Notes from 04/01 meeting:

The consensus of the core language working group was that it
should be permitted to declare constructors as friends.

Proposed Resolution (revised October 2002):

Change paragraph 1a in 3.4.3.1
class.qual (added by
the resolution of issue 147) as follows:

If the nested-name-specifier nominates a class C, and the
name specified after the nested-name-specifier, when looked up in
C, is the injected-class-name of C (clause
9
class), the name is instead considered to name the
constructor of class C. Such a constructor name shall be used
only in the declarator-id of a constructor definitiondeclaration that appears outside of the class definitionnames a constructor....

Note: the above does not allow qualified names to be used for
in-class declarations; see 8.3
dcl.meaning paragraph 1.
Also note that issue 318 updates the
same paragraph.

326.
Wording for definition of trivial constructor

In 12.1
class.ctor paragraph 5, the standard says
"A constructor is trivial if [...]", and goes on to define a trivial
default constructor. Taken literally, this would mean that a copy
constructor can't be trivial (contrary to 12.8
class.copy paragraph 6). I suggest changing this to "A default
constructor is trivial if [...]". (I think the change is purely
editorial.)

A default constructor for a class X
is a constructor of class X
that can be called without an argument.
If there is no
user-declareduser-declared
constructor for class X,
a default constructor is implicitly declared.
An
implicitly-declaredimplicitly-declared
default constructor
is an inline public member of its class.
A
default
constructor is trivial
if it is an implicitly-declared default constructor
and if:

all the direct base classes of its class
have trivial
default
constructors, and

for all the nonstatic data members of its class
that are of class type (or array thereof),
each such class has a trivial
default
constructor.

Otherwise, the
default
constructor is non-trivial.

Change 12.4
class.dtor paragraphs 3-4 as follows (the main
changes are removing italics):

If a class has no
user-declareduser-declared
destructor, a destructor is declared
implicitly. An
implicitly-declaredimplicitly-declared
destructor is an inline public
member of its class. A destructor is trivial if it is an
implicitly-declared destructor and if:

all of the direct base classes of its class have trivial destructors and

for all of the non-static data members of its class that are of
class type (or array thereof), each such class has a trivial
destructor.

331.
Allowed copy constructor signatures

A copy constructor for a class X is a constructor with a
first parameter of type X & or of type const X &. [Note:
see 12.8
class.copy for more information on copy constructors.]

No mention is made of constructors with first parameters of types
volatile X & or const volatile X &. This statement seems to be
in contradiction with 12.8
class.copy paragraph 2 which states

A non-template constructor for class X is a copy constructor
if its first parameter is of type X &, const X &, volatile X &
or const volatile X &, ...

12.8
class.copy paragraph 5 also mentions the volatile
versions of the
copy constructor, and the comparable paragraphs for copy assignment
(12.8
class.copy paragraphs 9 and 10) all allow volatile
versions, so it seems that 12.1
class.ctor is at fault.

86.
Lifetime of temporaries in query expressions

In
12.2
class.temporary
paragraph 5,
should binding a reference to the result of a "?" operation, each
of whose branches is a temporary, extend both temporaries?

Here's an example:

const SFileName &C = noDir ? SFileName("abc") : SFileName("bcd");

Do the temporaries created by the SFileName conversions survive
the end of the full expression?

Notes from 10/00 meeting:

Other problematic examples include cases where the temporary
from one branch is a base class of the temporary from the other (i.e.,
where the implementation must remember which type of temporary must
be destroyed), or where one branch is a temporary and the other is
not. Similar questions also apply to the comma operator. The sense
of the core language working group was that implementations should
be required to support these kinds of code.

Notes from the March 2004 meeting:

We decided that the cleanest model is one in which any "?" operation
that returns a class rvalue always copies one of its operands to
a temporary and returns the temporary as the result of the operation.
(Note that this may involve slicing.) An implementation would be
free to optimize this using the rules in 12.8
class.copy
paragraph 15, and in fact we would expect that in many cases
compilers would do such optimizations. For example, the compiler
could construct both rvalues in the above example into a
single temporary, and thus avoid a copy.

124.
Lifetime of temporaries in default initialization of class arrays

Jack Rouse:
12.2
class.temporary
states that temporary objects will normally be destroyed at the
end of the full expression in which they are created. This can create
some unique code generation requirements when initializing a class
array with a default constructor that uses a default argument. Consider
the code:

The full expression allocating the array in f(int) includes the
default constructor for S. Therefore according to
1.9
intro.execution paragraph 14, it
includes the default argument expression for S(int).
So evaluation of
the full expression should include evaluating the default argument "n"
times and creating "n" temporaries of type T. But the destruction of
the temporaries must be delayed until the end of the full expression
so this requires allocating space at runtime for "n" distinct
temporaries. It is unclear how these temporaries are supposed to be
allocated and deallocated. They cannot readily be autos because a
variable allocation is required.

I believe that many existing implementations will destroy the
temporaries needed by the default constructor after each array element
is initialized. But I can't find anything in the standard that allows
the temporaries to be destroyed early in this case.

I think the standard should allow the early destruction of temporaries
used in the default initialization of class array elements. I believe
early destruction is the status quo, and I don't think the users of
existing C++ compilers have been adversely impacted by it.

201.
Order of destruction of temporaries in initializers

According to 12.2
class.temporary
paragraph 4, an expression appearing as the initializer in an object
definition constitutes a context "in which temporaries are destroyed
at a different point than the end of the full-expression." It goes on
to say that the temporary containing the value of the expression
persists until after the initialization is complete (see also
issue 117). This seems to presume
that the end of the full-expression is a point earlier than the
completion of the initialization.

However, according to
1.9
intro.execution
paragraphs 12-13, the
full-expression in such cases is, in fact, the entire initialization.
If this is the case, the behavior described for temporaries in an
initializer expression is simply the normal behavior of temporaries in
any expression, and treating it as an exception to the general rule is
both incorrect and confusing.

If the initializer for an object or
sub-object is a full-expression, the initialization of
the object or sub-object (e.g., by calling a constructor
or copying an expression value) is considered to be part
of the full-expression.

There are two contexts in which temporaries are
destroyed at a different point than the end of the
full-expression. The first context is when a
default constructor is called to initialize an
element of an array. If the constructor has one
or more default arguments, any temporaries created
in the default argument expressions are destroyed
immediately after return from the constructor.

a=f(a) requires a temporary for either the argument a
or the result of f(a) to avoid undesired aliasing of a.

The note seems to imply that an implementation is allowed to omit
copying "a" to f's formal argument, or to omit using a temporary for
the return value of f. I don't find that license in normative text.

Function f returns an X by value, and in the expression the value is
assigned (not copy-constructed) to "a". I don't see how that
temporary can be omitted. (See also 12.8
class.copy p 15)

Since "a" is an lvalue and not a temporary, I don't see how copying
"a" to f's formal parameter can be avoided.

Section 12.2
class.temporary paragraph 3 says a temp is
destroyed as the last step in
evaluating the full expression. But the expression C().get() has a
reference type. Does 12.2
class.temporary paragraph 3 require
that the dereference to get a
boolean result occur before the destructor runs, making the code
valid? Or does the code have undefined behavior?

Bill Gibbons:
It has undefined behavior, though clearly this wasn't intended.
The lvalue-to-rvalue conversion that occurs in the "if" statement is
not currently part of the full-expression.

A full-expression is an expression that is not a subexpression
of another expression. If a language construct is defined to
produce an implicit call of a function, a use of the language
construct is considered to be an expression for the purposes
of this definition.

The note in section 1.9
intro.execution paragraph 12 goes
on to explain that this covers
expressions used as initializers, but it does not discuss
lvalues within temporaries.

It is a small point but it is probably worth correcting
1.9
intro.execution paragraph 12.
Instead of the "implicit call of a function" wording, it might
be better to just say that a full-expression includes any implicit
use of the expression value in the enclosing language construct,
and include a note giving implicit calls and lvalue-to-rvalue
conversions as examples.

A full-expression is an expression that is not a
subexpression of another expression. If a language construct is
defined to produce an implicit call of a function, a use of the
language construct is considered to be an expression for the
purposes of this definition. Conversions applied to the result of
an expression in order to satisfy the requirements of the language
construct in which the expression appears are also considered to be
part of the full-expression.

[Note: certain contexts in C++ cause the evaluation
of a full-expression that results from a syntactic construct other
than expression (5.18
expr.comma). For example,
in 8.5
dcl.init one syntax for
initializer is

( expression-list )

but the resulting construct is a function call upon a constructor
function with expression-list as an argument list; such a
function call is a full-expression. For example, in
8.5
dcl.init, another
syntax for initializer is

= initializer-clause

but again the resulting construct might be a function call upon a
constructor function with one assignment-expression as an
argument; again, the function call is a full-expression. ][Example:

443.
Wording nit in description of lifetime of temporaries

There seems to be a typo in
12.2
class.temporary/5, which says "The temporary to which
the reference is bound or the temporary that is the complete object TO a
subobject OF which the TEMPORARY is bound persists for the lifetime of the
reference except as specified below."

I think this should be "The temporary to which the reference is bound or the
temporary that is the complete object OF a subobject TO which the REFERENCE
is bound persists for the lifetime of the reference except as specified
below."

The temporary to which the reference is bound or the temporary
that is the complete object toof a subobject
ofto which the temporaryreference
is bound persists for the lifetime of the reference except as
specified below.

464.
Wording nit on lifetime of temporaries to which references are bound

[...] if obj2 is an object with static or automatic
storage duration created after the temporary is
created, the temporary shall be destroyed after
obj2 is destroyed.

For the temporary to be destroyed after obj2 is destroyed, when obj2 has
static storage, I would say that the reference to the temporary should
also have static storage, but that is IMHO not clear from the
paragraph.

Example:

void f ()
{
const T1& ref = T1();
static T2 obj2;
...
}

Here the temporary would be destoyed before obj2, contrary to the
rule above.

Steve Adamczyk: I agree there's a minor issue here. I think
the clause quoted above meant for obj1 and obj2 to have the same storage
duration. Replacing "obj2 is an object with static or automatic
storage duration" by "obj2 is an object with the same storage duration as
obj1" would, I believe, fix the problem.

... In addition, the destruction of temporaries bound to references
shall take into account the ordering of destruction of objects with
static or automatic storage duration (3.7.1
basic.stc.static,
3.7.2
basic.stc.auto); that is, if obj1 is an object
with static or automatic storage duration created before the
temporary is created with the same storage duration as the
temporary, the temporary shall be destroyed before obj1
is destroyed; if obj2 is an object with static or
automatic storage duration created after the temporary is
created with the same storage duration as the temporary, the
temporary shall be destroyed after obj2 is destroyed...

296.
Can conversion functions be static?

May user-defined conversion functions be static? That is, should this
compile?

class Widget {
public:
static operator bool() { return true; }
};

All my compilers hate it. I hate it, too. However, I don't see anything
in 12.3.2
class.conv.fct that makes it illegal. Is this a
prohibition that arises from
the grammar, i.e., the grammar doesn't allow "static" to be followed by a
conversion-function-id in a member function declaration? Or am I just
overlooking something obvious that forbids static conversion functions?

I believe that clause 3 is correct and that clause 12 is in error.
We worked hard to get the destructor lookup rules in clause 3 to be
right, and I think we failed to notice that a change was also needed
in clause 12.

Mike Miller:

Unfortunately, I don't believe 3.4.3
basic.lookup.qual covers
the case of p->AB::~AB(). It's clearly intended to do so, as
evidenced by 3.4.3.1
class.qual paragraph 1 ("a
destructor name is looked up as specified in 3.4.3
basic.lookup.qual"), but I don't think the language there does so.

The relevant paragraph is 3.4.3
basic.lookup.qual paragraph
5. (None of the other paragraphs in that section deal with this topic
at all.) It has two parts. The first is

If a pseudo-destructor-name (5.2.4
expr.pseudo)
contains a nested-name-specifier, the type-names are
looked up as types in the scope designated by the
nested-name-specifier.

This sentence doesn't apply, because ~AB isn't a
pseudo-destructor-name. 5.2.4
expr.pseudo makes
clear that this syntactic production (5.2
expr.post
paragraph 1) only applies to cases where the type-name is not a
class-name. p->AB::~AB is covered by the production
using id-expression.

where the nested-name-specifier designates a namespace name,
and in a qualified-id of the form:

::optnested-name-specifier class-name::~class-name

the class-names are looked up as types in the scope
designated by the nested-name-specifier.

This wording doesn't apply, either. The first one doesn't because
the nested-name-specifier is a class-name, not a
namespace name. The second doesn't because there's only one layer of
qualification.

As far as I can tell, there's no normative text that specifies how
the ~AB is looked up in p->AB::~AB().
3.4.3.1
class.qual, where all the other
class member qualified lookups are handled, defers to
3.4.3
basic.lookup.qual, and 3.4.3
basic.lookup.qual
doesn't cover the case.

Jason Merrill:
My thoughts on the subject were that the name we use in a destructor call
is really meaningless; as soon as we see the ~ we know what the
user means,
all we're doing from that point is testing their ability to name the
destructor in a conformant way. I think that everyone will agree that

anything::B::~B()

should be well-formed, regardless of the origins of the name "B". I
believe that the rule about looking up the second "B" in the same context
as the first was intended to provide this behavior, but to me this seems
much more heavyweight than necessary. We don't need a whole new type of
lookup to be able to use the same name before and after the ~;
we can just
say that if the two names match, the call is well-formed. This is
significantly simpler to express, both in the standard and in an
implementation.

Anyone writing two different names here is either deliberately writing
obfuscated code, trying to call the destructor of a nested class, or
fighting an ornery compiler (i.e. one that still wants to see
B_alias::~B()). I think we can ignore the first case.
The third would be
handled by reverting to the old rule (look up the name after ~ in the
normal way) with the lexical matching exception described above -- or we
could decide to break such code, do no lookup at all, and only accept a
matching name. In a good implementation, the second should probably get an
error message telling them to write Outer::Inner::~Inner instead.

We discussed this at the meetings, but I don't remember if we came to
any sort of consensus on a direction. I see three options:

Stick with the status quo, i.e. the special lookup rule such that if the
name before ::~ is a class name, the name after ::~
is looked up in the
same scope as the previous one. If we choose this option, we just need
better wording that actually expresses this, as suggested in the issue
list. This option breaks old B_alias::~B code where B_alias
is declared in a different scope from B.

Revert to the old rules, whereby the name after ::~ is looked up just
like a name after ::, with the exception that if it matches the name
before ::~ then it is considered to name the same class. This option
supports old code and code that writes B_alias::~B_alias. It does not
support the q->I1::~I2 usage of 3.4.3
basic.lookup.qual,
but that seems like deliberate
obfuscation. This option is simpler to implement than #1.

Do no lookup for a name after ::~; it must match the
name before. This
breaks old code as #1, but supports the most important case where the
names match. This option may be slightly simpler to implement than #2.
It is certainly easier to teach.

My order of preference is 2, 3, 1.

Incidentally, it seems to me oddly inconsistent to allow
Namespace::~Class,
but not Outer::~Inner.
Prohibiting the latter makes sense from the
standpoint of avoiding ambiguity, but what was the rationale for allowing
the former?

John Spicer:
I agree that allowing Namespace::~Class is odd.
I'm not sure where this
came from. If we eliminated that special case, then I believe the #1
rule would just be that in A::B1::~B2 you look up B1
and B2 in the same place in all cases.

I don't like #2. I don't think the "old" rules represent a deliberate
design choice, just an error in the way the lookup was described. The
usage that rule permits p->X::~Y
(where Y is a typedef to X defined in X),
but I doubt people really do that. In other words, I think that #1
a more useful special case than #2 does, not that I think either special
case is very important.

One problem with the name matching rule is handling cases like:

A<int> *aip;
aip->A<int>::~A<int>(); // should work
aip->A<int>::~A<char>(); // should not

I would favor #1, while eliminating the special case of
Namespace::~Class.

Proposed resolution (10/01):

Replace the normative text of 3.4.3
basic.lookup.qual paragraph 5
after the first sentence with:

252.
Looking up deallocation functions in virtual destructors

There is a mismatch between 12.4
class.dtor
paragraph 11 and 12.5
class.free paragraph 4 regarding
the lookup of deallocation functions in virtual destructors.
12.4
class.dtor says,

At the point of definition of a virtual destructor (including an
implicit definition (12.8
class.copy)), non-placement
operator delete shall be looked up in the scope of the destructor's
class (3.4.1
basic.lookup.unqual) and if found shall be accessible
and unambiguous. [Note: this assures that an operator delete
corresponding to the dynamic type of an object is available for the
delete-expression (12.5
class.free). ]

The salient features to note from this description are:

The lookup is "in the scope of the destructor's class," which
implies that only members are found (cf 12.2
class.temporary).
(The cross-reference would indicate otherwise, however, since it
refers to the description of looking up unqualified names; this kind
of lookup "spills over" into the surrounding scope.)

Only non-placement operator delete is looked up. Presumably
this means that a placement operator delete is ignored in the
lookup.

If a delete-expression begins with a unary ::
operator, the deallocation function's name is looked up in global
scope. Otherwise, if the delete-expression is used to
deallocate a class object whose static type has a virtual destructor,
the deallocation function is the one found by the lookup in the
definition of the dynamic type's virtual destructor (12.4
class.dtor). Otherwise, if the delete-expression is used to
deallocate an object of class T or array thereof, the static
and dynamic types of the object shall be identical and the
deallocation function's name is looked up in the scope of
T. If this lookup fails to find the name, the name is looked
up in the global scope. If the result of the lookup is ambiguous or
inaccessible, or if the lookup selects a placement deallocation
function, the program is ill-formed.

Points of interest in this description include:

For a class type with a virtual destructor, the lookup is
described as being "in the definition of the dynamic type's
virtual destructor," rather than "in the scope of the dynamic
type." That is, the lookup is assumed to be an unqualified
lookup, presumably terminating in the global scope.

The assumption is made that the lookup in the virtual
destructor was successful ("...the one found...", not "...the
one found..., if any"). This will not be the case if
the deallocation function was not declared as a member somewhere
in the inheritance hierarchy.

The lookup in the non-virtual-destructor case does
find placement deallocation functions and can fail as a result.

Suggested resolution: Change the description of the
lookup in 12.4
class.dtor paragraph 11 to match the
one in 12.5
class.free paragraph 4.

At the point of definition of a virtual destructor (including an
implicit definition), the non-array deallocation function is looked up
in the scope of the destructor's class (10.2
class.member.lookup),
and, if no declaration is found, the function is looked up in the
global scope. If the result of this lookup is ambiguous or
inaccessible, or if the lookup selects a placement deallocation
function, the program is ill-formed. [Note: this assures that
a deallocation function corresponding to the dynamic type of an object
is available for the delete-expression (12.5
class.free).]

272.
Explicit destructor invocation and qualified-ids

an explicit destructor call must always be written using a member
access operator (5.2.5
expr.ref); in particular, the
unary-expression~X() in a member function is not an
explicit destructor call (5.3.1
expr.unary.op).

This note is incorrect, as an explicit destructor call can be
written as a qualified-id, e.g., X::~X(), which
does not use a member access operator.

[Note: an explicit destructor call must always be written
using a member access operator (5.2.5
expr.ref)
or a qualified-id (5.1
expr.prim);
in particular, the unary-expression~X() in a
member function is not an explicit destructor call
(5.3.1
expr.unary.op).]

510.
Default initialization of POD classes?

8.5
dcl.init paragraph 10 makes it clear that
non-static POD class objects with no initializer are left uninitialized
and have an indeterminate initial value:

If no initializer is specified for an object, and the object is of
(possibly cv-qualified) non-POD class type (or array thereof), the
object shall be default-initialized; if the object is of
const-qualified type, the underlying class type shall have a
user-declared default constructor. Otherwise, if no initializer is
specified for a non-static object, the object and its subobjects, if
any, have an indeterminate initial value; if the object or any of
its subobjects are of const-qualified type, the program is ill-formed.

12.6
class.init paragraph 1, however, implies that all
class objects without initializers, whether POD or not, are
default-initialized:

When no initializer is specified for an object of (possibly
cv-qualified) class type (or array thereof), or the initializer has
the form (), the object is initialized as specified in
8.5
dcl.init. The object is default-initialized if there
is no initializer, or value-initialized if the initializer
is ().

When no initializer is specified for an object of (possibly
cv-qualified) class type (or array thereof), or the initializer has
the form (), the object is initialized as specified in
8.5
dcl.init. The object is default-initialized if
there is no initializer, or value-initialized if the initializer
is ().

is written, the set of overloaded functions named by C::f
must not contain
any nonstatic member functions. A footnote gives the rationale: if a member
of C::f is a nonstatic member function,
&C::f is a pointer to member
constant, and therefore the call is invalid.

This is clear, it's implementable, and it doesn't directly contradict
anything else in the standard. However, I'm not sure it's consistent
with some similar cases.

In 13.4
over.over
paragraph 5,
second example, it is made amply clear that when &C::f is
used as the address of a function, e.g.,

int (*pf)(int) = &C::f;

the overload set can contain both static and nonstatic member functions.
The function with the matching signature is selected, and if it is
nonstatic &C::f
is a pointer to member function, and otherwise &C::f is
a normal pointer to function.

is a valid call even if the overload set contains both static and
nonstatic member functions. Overload resolution is done, and if a
nonstatic member function is selected, an implicit this-> is added,
if that is possible.

Those paragraphs seem to suggest the general rule that you do overload
resolution first and then you interpret the construct you have
according to the function selected. The fact that there are static
and nonstatic functions in the overload set is irrelevant; it's only
necessary that the chosen function be static or nonstatic to match
the context.

Given that, I think it would be more consistent if the
(&C::f)() case
would also do overload resolution first. If a nonstatic member is
chosen, the program would be ill-formed.

The fourth case arises from a postfix-expression of the form
&F, where
F names a set of overloaded functions.
In the context of a function call,
the set of functions named by F shall contain only non-member
functions and static member functions. [Footnote:
If F names a non-static member function, &F
is a pointer-to-member, which cannot be used with the function call syntax.]
And in this context using &F behaves the same as using&F is treated the same as
the name F by itself.
Thus, (&F)(expression-listopt)
is simply
(F)(expression-listopt), which is
discussed in 13.3.1.1.1
over.call.func.
If the function selected by overload resolution according to
13.3.1.1.1
over.call.func is
a nonstatic member function, the program is
ill-formed. [Footnote: When F is a nonstatic member
function, a reference of the form &A::F is a
pointer-to-member,
which cannot be used with the function-call syntax, and
a reference of the form &F is an invalid use of the
"&" operator on a nonstatic member function.]
(The resolution of &F in other contexts is described in
13.4
over.over.)

If the ordinary unqualified lookup of the name finds the declaration
of a class member function, the associated namespaces and classes are
not considered.

to

If the ordinary unqualified lookup of the name finds the declaration
of a class member function, or a block-scope function declaration
that is not a using-declaration, the associated namespaces
and classes are not considered.

Mark Mitchell:
It looks to me like G++ is correct, given 13.3.1.1.1
over.call.func.
This case is the "unqualified function call" case described in paragraph
3 of that section. ("Unqualified" here means that there is no "x->" or
"x." in front of the call, not that the name is unqualified.)

That paragraph says that you first do name lookup. It then asks you
to look at what declaration is returned. (That's a bit confusing; you
presumably get a set of declarations. Or maybe not; the name lookup
section says that if name lookup finds a non-static member in a context
like this the program is in error. But surely this program is not
erroneous. Hmm.)

Anyhow, you have -- at least -- "S::f(char)" as the result of the
lookup.

The keyword "this" is not in scope, so "all overloaded declarations of
the function name in T become candidate functions and a contrived object
of type T becomes the implied object argument." That means we get
both versions of "f" at this point. Then, "the call is ill-formed,
however, if overload resolution selects one of the non-static members
of T in this case." Since, in this case, "S::f(char)" is the winner,
the program is ill-formed.

Steve Adamczyk:
This result is surprising, because we've selected a function
that we cannot call, when there is another function that can be
called. This should either be ambiguous, or it should select the static
member function. See also 13.3.1
over.match.funcs paragraph 2:
"Similarly, when
appropriate, the context can construct an argument list that
contains an implied object argument..."

Notes from October 2002 meeting:

We agreed that g++ has it right, but the standard needs to be clearer.

In qualified function calls, the name to be resolved is an
id-expression and is preceded by an -> or .
operator. Since the construct A->B is generally equivalent
to (*A).B, the rest of clause 13
over
assumes, without loss of generality, that all member function calls have been
normalized to the form that uses an object and the . operator.
Furthermore, clause 13
over
assumes that the postfix-expression that is the left
operand of the . operator has type ``cvT''
where T denotes a class.
[Footnote: Note that cv-qualifiers on the type of
objects are significant in overload resolution for both lvalue and
class rvalue objects. --- end footnote]
Under this assumption, the id-expression in the
call is looked up as a member function of T following the rules for
looking up names in classes (10.2
class.member.lookup).
If a member function is found, that function and its overloaded
declarationsThe function declarations found by that lookup
constitute the set of candidate functions. The argument list is the
expression-list in the call augmented by the addition of the
left operand of the . operator in the normalized member
function call as the implied object argument (13.3.1
over.match.funcs).

In unqualified function calls, the name is
not qualified by an -> or . operator and has the more
general form of a primary-expression. The name is looked up in the
context of the function call following the normal rules for name lookup in
function calls (3.4.2
basic.lookup.argdep
3.4
basic.lookup).
If the name resolves to a non-member function declaration, that
function and its overloaded declarationsThe function declarations found by that lookup
constitute the set of candidate functions.
[Footnote: Because of the usual name hiding rules,
these will be introduced by declarations or by using-directives all
found in the same block or all found at namespace scope. --- end footnote]Because of the rules for name lookup,
the set of candidate functions consists (1) entirely of non-member functions
or (2) entirely of member functions of some class T. In
case (1), tThe argument list is the same as
the expression-list in the call.
If the name resolves to a nonstatic member function, then the
function call is actually a member function call.In case (2), the argument list is the expression-list in the
call augmented by the addition of an implied object argument as in
a qualified function call.
If the keyword this (9.3.2
class.this)
is in scope and refers to the class Tof that member
function, or a derived class thereofof T, then
the function call is transformed into a normalized qualified
function call usingimplied object argument is(*this)as the
postfix-expression
to the left of the . operator. The candidate
functions and argument list are as described for qualified function calls
above. If the keyword this is not in scope or refers to
another class, then name resolution found a static member of some
class T.
In this case,all overloaded declarations of the function name in
T become candidate functions and a contrived object of
type T becomes the implied object argument.
[Footnote: An implied object argument must be
contrived to correspond to the implicit object parameter attributed to member
functions during overload resolution. It is not used in the call to the
selected function. Since the member functions all have the same implicit
object
parameter, the contrived object will not be the cause to select or reject a
function. --- end footnote]
If the argument list is augmented by a contrived object
and The call is ill-formed, however, if overload
resolution selects one of the non-static member functions of
T, the
call is ill-formed in this case.

280.
Access and surrogate call functions

According to 13.3.1.1.2
over.call.object paragraph 2, when
the primary-expressionE in the function call syntax
evaluates to a class object of type "cvT", a
surrogate call function corresponding to an appropriate conversion
function declared in a direct or indirect base class B of
T is included or not included in the set of candidate
functions based on class B being accessible.

For instance in the following code sample, as per the paragraph in
question, the expression c(3) calls f2, instead of
the construct being ill-formed due to the conversion function
A::operator fp1 being inaccessible and its corresponding
surrogate call function providing a better match than the surrogate
call function corresponding to C::operator fp2:

The fact that the accessibility of a base class influences the
overload resolution process contradicts the fundamental language rule
(3.4
basic.lookup paragraph 1, and 13.3
over.match paragraph 2) that access checks are applied only once
name lookup and function overload resolution (if applicable) have
succeeded.

Similarly, surrogate call functions are added to the set of candidate
functions for each conversion function declared in an accessible base
class provided the function is not hidden within T by another
intervening declaration.

with

Similarly, surrogate call functions are added to the set of candidate
functions for each conversion function declared in a base class of
T provided the function is not hidden within T by
another intervening declaration.

If such a surrogate call function is selected by overload resolution,
its body, as defined above, will be executed to convert E to the
appropriate function and then to invoke that function with the
arguments of the call.

by

If such a surrogate call function is selected by overload resolution,
the corresponding conversion function will be called to convert
E to the appropriate function pointer or reference,
and the function will then be invoked with the arguments of the call.
If the conversion function cannot be called
(e.g., because of an ambiguity), the program is ill-formed.

required to be diagnosed because of the op<<? The reason
being that the class may actually have an op<<(const char *) in it.

What is it? un- or ill-something? Diagnosable? No problem at all?

Steve Adamczyk:
I don't know of any requirement in the standard that the class be complete.
There is a rule that will instantiate a class template in order to be able
to see whether it has any operators. But I wouldn't think one wants to
outlaw the above example merely because the user might have an
operator<<
in the class; if he doesn't, he would not be pleased that the above
is considered invalid.

Mike Miller:
Hmm, interesting question. My initial reaction is that it just
uses ::operator<<; any A::operator<<
simply won't be considered in
overload resolution. I can't find anything in the Standard that
would say any different.

The closest analogy to this situation, I'd guess, would be
deleting a pointer to an incomplete class;
5.3.5
expr.delete paragraph 5 says that that's
undefined behavior if the complete type has a non-trivial destructor
or an operator delete. However, I tend to think that that's because
it deals with storage and resource management, not just because it
might have called a different function. Generally, overload
resolution that goes one way when it might have gone another with
more declarations in scope is considered to be not an error, cf
7.3.3
namespace.udecl paragraph 9,
14.6.3
temp.nondep paragraph 1, etc.

So my bottom line take on it would be that it's okay, it's up to
the programmer to ensure that all necessary declarations are in
scope for overload resolution. Worst case, it would be like the
operator delete in an incomplete class -- undefined behavior, and
thus not required to be diagnosed.

13.3.1.2
over.match.oper paragraph 3, bullet 1, says,
"If T1 is a class type, the set of member candidates is the result
of the qualified lookup of T1::operator@
(13.3.1.1.1
over.call.func)." Obviously, that lookup is
not possible if T1 is
incomplete. Should
13.3.1.2
over.match.oper paragraph 3, bullet 1, say
"complete class type"? Or does the inability to perform
the lookup mean that the program is ill-formed?
3.2
basic.def.odr paragraph 4 doesn't apply,
I don't think, because you don't know whether you'll be applying a
class member access operator until you know whether the operator
involved is a member or not.

Notes from October 2003 meeting:

We noticed that the title of this issue did not match the
body. We checked the original source and then corrected the title
(so it no longer mentions templates).

We decided that this is similar to other cases like
deleting a pointer to an incomplete class, and it should not be
necessary to have a complete class. There is no undefined
behavior.

60.
Reference binding and valid conversion sequences

Does dropping a cv-qualifier on a reference binding prevent the binding
as far as overload resolution is concerned?
Paragraph 4 says "Other restrictions
on binding a reference to a particular argument do not affect the formation
of a conversion sequence." This was intended to refer to things like access
checking, but some readers have taken that to mean that any aspects of
reference binding not mentioned in this section do not preclude the binding.

Other restrictions on binding a reference to a particular argument that
are not based on the types of the reference and the argument do
not affect the formation of a standard conversion sequence, however.

The question is whether &f<int> identifies a unique function.
&g<int> is clearly ambiguous.

13.4
over.over
paragraph 1
says that a function template name is considered to name a
set of overloaded functions. I believe it should be expanded to say that
a function template name with an explicit template argument list is also
considered to name a set of overloaded functions.

In the general case, you need to have a destination type in order
to identify a unique function. While it is possible to permit this, I
don't think it is a good idea because such code depends on there only
being one template of that name that is visible.

The EDG front end issues an error on this use of "f".
egcs 1.1.1 allows
it, but the most current snapshot of egcs that I have also issues an error
on it.

It has been pointed out that when dealing with nontemplates, the rules
for taking the address of a single function differ from the rules
for an overload set, but this asymmetry is needed for C compatibility.
This need does not exist for the template case.

My feeling is that a general rule is better than a general rule plus
an exception. The general rule is that you need a destination type
to be sure that the operation will succeed. The exception is when
there is only one template in the set and only then when you provide
values for all of the template arguments.

It is true that in some cases you can provide a shorthand, but only if
you encourage a fragile coding style (that will cause programs to break
when additional templates are added).

I think the standard needs to specify one way or the other how this
case should be handled. My recommendation would be that it is
ill-formed.

If John's recommendation is adopted, this code will become
ill-formed. I bet there will be a lot of explanation for users
necessary why this fails and that they have to change
add<10> to something like
(int (*)(int))add<10>.

This example code is probably common practice because this
use of the STL is typical and is accepted in many current
implementations. I strongly urge that this issue be resolved
in favor of keeping this code valid.

Bill Gibbons: I find this rather surprising. Shouldn't
a template-id which specifies all of the template arguments
be treated like a declaration-only explicit instantiation,
producing a set of ordinary function declarations? And when that
set happens to contain only one function, shouldn't the example
code work?

The consensus of the group was that the add example
should not be an error.

Proposed resolution (October 2002):

In 13.4 add to the end of paragraph 2:

[Note: As described in 14.8.1
temp.arg.explicit,
if deduction fails and the
function template name is followed by an explicit template
argument list, the template-id is then examined to
see whether it identifies a single function template
specialization. If it does, the template-id is
considered to be an lvalue for that function template
specialization. The target type is not used in that
determination.]

In contexts where deduction is done and fails,
or in contexts where deduction is not done,
if a template argument list is specified and it, along with any
default template arguments, identifies a single function template
specialization, then the template-id is an lvalue
for the function template specialization.

221.
Must compound assignment operators be member functions?

Is the intent of 13.5.3
over.ass paragraph 1 that
all assignment operators be non-static member functions
(including operator+=, operator*=, etc.) or only
simple assignment operators (operator=)?

Notes from 04/00 meeting:

Nearly all references to "assignment operator" in the IS mean
operator= and not the compound assignment operators. The ARM
was specific that this restriction applied only to operator=.
If it did apply to compound assignment operators, it would be
impossible to overload these operators for bool operands.

Proposed resolution (04/01):

Change the title of 5.17
expr.ass from "Assignment
operators" to "Assignment and compound assignment operators."

There are several assignment operators, all of which group
right-to-left. All require a modifiable lvalue as their left operand,
and the type of an assignment expression is that of its left operand.
The result of the assignment operation is the value stored in the left
operand after the assignment has taken place; the result is an lvalue.

to

The assignment operator (=) and the compound assignment
operators all group right-to-left. All require a modifiable lvalue as
their left operand and return an lvalue with the type and value of
the left operand after the assignment has taken place.

Additional note (10/00): Paragraphs 2-6 of
5.17
expr.ass should all be understood to apply to
simple assignment only and not to compound assignment operators.

(This then is a copy of the two lines in 5.2
expr.post
covering "->dtor")

Alternatively remove the sentence "It implements class member
access using ->" and the syntax line following.

Reasons:

Currently stdc++ is inconsistent when handling expressions of
the form "postfixexpression->scalar_type_dtor()":
If "postfixexpression" is a pointer to the scalar type, it is OK,
but if "postfixexpression" refers to any smart pointer class
(e.g. iterator or allocator::pointer) with class specific
CLASS::operator->() returning pointer to the scalar type, then
it is ill-formed; so while c++98 does allow CLASS::operator->()
returning pointer to scalar type, c++98 prohibits any '->'-expression
involving this overloaded operator function.

Not only is this behaviour inconsistent, but also when
comparing the corresponding chapters of c++pl2 and stdc++98
it looks like an oversight and unintended result.
Mapping between stdc++98 and c++pl2:

For the single line of c++pl2.r.5.2 covering "->dtor",
5.2 [expr.post] has two lines.
Analogously c++pl2.r.5.2.4 has been doubled to 5.2.4 [expr.pseudo]
and 5.2.5 [expr.ref].
From 13.5.6 [over.ref], the sentence forbiding CLASS::operator->()
returning pointer to scalar type has been removed.
Only the single line of c++pl2.r.13.4.6 (<-> c++pl2.r.5.2's
single line) has not gotten its 2nd line when converted
into 13.5.6 [over.ref].

Additionally GCC32 does is right (but against 13.5.6 [over.ref]).

AFAICS this would not break old code except compilers like VC7x
and Comeau4301.

It does not add new functionality, cause any expression
class_type->scalar_type_dtor() even today can be substituted
through (*class_type).scalar_type_dtor().

Without this fix, template functions like
some_allocator<T>::destroy(p)
must use "(*p).~T()" or "(*p).T::~T()" when calling the destructor,
otherwise the simpler versions "p->~T()" or "p->T::~T()"
could be used.

An expression x->m is interpreted
as (x.operator->())->m for a class object x of
type T if T::operator->() exists and if the
operator is selected as the best match function by the overload
resolution mechanism (13.3
over.match).

425.
Set of candidates for overloaded built-in operator with float operand

The code is compiled without errors or warnings from EDG-based compilers
(Comeau, Intel), but rejected from others (GCC, MSVC [7.1]). The
question: Who is correct? Where should I file the bug report?

To explain the problem: The EDG seems to see 1.0f*a as a call to the
unambiguous operator*(float,float) and thus casts 'a' to 'float'. The
other compilers have several operators (float*float, float*double,
float*int, ...) available and thus can't decide which cast is
appropriate. I think the latter is the correct behaviour, but I'd like
to hear some comments from the language lawyers about the standard's
point of view on this problem.

Since your first argument is of type float, the (float, float) case is
preferred
over the (double, double) case (the latter would require a floating-point
promotion).

Stave Adamczyk:
Daveed's statement is wrong; as Andreas says, the prototypes in
13.6
over.built paragraph 12
have pairs of types, not the same type twice. However, the list of
possibilities considered in Andreas' message is wrong also:
13.6
over.built paragraph 12
calls for pairs of promoted arithmetic types, and float is not
a promoted type (it promotes to double -- see
4.6
conv.fpprom).

Nevertheless, the example is ambiguous. Let's look at the overload
resolution costs. The right operand is always going to have a
user-defined-conversion cost (the template conversion function
will convert directly to the const version of the second
parameter of the prototype). The left operand is always going to
have a promotion (float --> double) or a standard conversion
(anything else). So the cases with promotions are better
than the others. However, there are several of those
cases, with second parameters of type int, unsigned int, long,
unsigned long, double, and long double, and all of those are
equally good. Therefore the example is ambiguous.

I can't tell if that's "promoted integral types plus (all) floating
types" or "promoted integral types plus (promoted) floating types".
I thought the latter was intended, but indeed the usual arithmetic
conversions could give you "float + float", so it makes sense that
float would be one of the possibilities. We should discuss this to
make sure everyone has the same interpretation.

204.
Exported class templates

14
temp
paragraph 7 allows class
templates to be declared exported, including member classes and member
class templates (implicitly by virtue of exporting the
containing template class). However, paragraph 8 does not exclude
exported class templates from the statement that

An exported template need only be declared (and not necessarily
defined) in a translation unit in which it is instantiated.

This is an incorrect implication; however, it is also not dispelled in
14.7.1
temp.inst
paragraph 6:

If an implicit instantiation of a class template specialization is
required and the template is declared but not defined, the program is
ill-formed.

This wording says nothing about the translation unit in which the
definition must be provided. Contrast this with
14.7.2
temp.explicit
paragraph 3:

A definition of a class template or a class member template shall be
in scope at the point of the explicit instantiation of the class
template or class member template.

John Spicer opined that even though 14
temp
paragraph 7 speaks of "declaring a class template exported," that does
not mean that the class template is "an exported template" in the
sense of paragraph 8. He suggested clarifying paragraph 7 to that
effect instead of the change to paragraph 8 suggested above, and
questioned the need for a change to 14.7.1
temp.inst.

A template-declaration may be preceded by the export keyword.
Such a template is said to be exported.
Declaring exported a class template is equivalent to declaring
exported all of its non-inline member functions, static data members,
member classes, member class templates, and non-inline member function
templates.

If a template is exported in one translation unit, it
shall be exported in all translation units in which
it appears; no diagnostic is required.
A declaration of an exported template shall appear with the export
keyword before any point of instantiation (14.6.4.1
temp.point)
of that template in that translation unit. In addition, the first
declaration of an exported template containing the export keyword
must not follow the definition of that template. The export
keyword shall not be used in a friend declaration.

Templates defined in an unnamed namespace, inline functions, and
inline function templates shall not be exported.
An exported non-class template shall be defined only once in a program;
no diagnostic is required. An exported non-class template
need only be declared (and not necessarily defined) in a translation
unit in which it is instantiated.

A non-exported non-class template must be defined in every translation unit
in which it is implicitly instantiated (14.7.1
temp.inst),
unless the corresponding
specialization is explicitly instantiated (14.7.2
temp.explicit)
in some translation unit; no diagnostic is required.

335.
Allowing export on template members of nontemplate classes

The syntax for "export" permits it only on template declarations.
Clause 14
temp paragraph 6 further restricts "export"
to appear only
on namespace scope declarations. This means that you can't export
a member template of a non-template class, as in:

class A {
template <class T> void f(T);
};

You can, of course, put export on the definition:

export template <class T> void A<T>::f(T){}

but in order for the template to be used from other translation units
(the whole point of export) the declaration in the other translation
unit must also be declared export.

There is also the issue of whether or not we should permit this usage:

export struct A {
template <class T> void f(T);
};

My initial reaction is to retain this prohibition as all current uses
of "export" are preceding the "template" keyword.

If we eliminate the requirement that "export" precede "template" there
is a similar issue regarding this case, which is currently prohibited:

534.
template-names and operator-function-ids

Taken literally, 14
temp paragraph 2 does not
permit operator functions to be templates:

In a function template declaration, the declarator-id shall be a
template-name (i.e., not a template-id).

and, in 14.2
temp.names paragraph 1, a template-name
is defined to be simply an identifier.

Issue 301 considered and rejected the
idea of changing the definition of template-name to include
operator-function-ids and conversion-function-ids.
Either that decision should be reconsidered or the various references
in the text to template-name should be examined to determine
if they should also mention the non-identifier possibilities
for function template names.

184.
Default arguments in template template-parameters

John Spicer:
Where can default values for the template parameters of
template template parameters be specified and where so they apply?

For normal template parameters, defaults can be specified only in
class template declarations and definitions, and they accumulate
across multiple declarations in the same way that function default
arguments do.

I think that defaults for parameters of template template parameters should
be handled differently, though. I see no reason why such a default should
extend beyond the template declaration with which it is associated.
In other words, such defaults are a property of a specific template
declaration and are not part of the interface of the template.

Gabriel Dos Reis:
On the other hand I fail to see the reasons why we should introduce
yet another special rule to handle that situation differently. I
think we should try to keep rules as uniform as possible.
For default values, it has been the case that one
should look for any declaration specifying default values. Breaking
that rules doesn't buy us anything, at least as far as I can see.
My feeling is that [allowing different defaults in different
declarations] is very confusing.

Mike Miller:
I'm with John on this one. Although we don't have the
concept of "prototype scope" for template parameter lists,
the analogy with function parameters would suggest that the
two declarations of T (in the template class definition and
the template member function definition) are separate
declarations and completely unrelated. While it's true that
you accumulate default arguments on top-level declarations in
the same scope, it seems to me a far leap to say that we ought
also to accumulate default arguments in nested declarations.
I would expect those to be treated as being in different
scopes and thus not to share default argument information.

When you look up the name T in the definition of
A<T>::f(),
the declaration you find has no default argument for the
parameter of T, so T<> should not be allowed.

Proposed Resolution (revised October 2002):

In 14.1
temp.param, add the following as a new paragraph
at the end of this section:

A template-parameter of a template template-parameter
is permitted to have a default template-argument. When such
default arguments are specified, they apply to the template
template-parameter in the scope of the template
template-parameter. [Example:

215.
Template parameters are not allowed in nested-name-specifiers

According to 14.1
temp.param paragraph 3, the following
fragment is ill-formed:

template <class T>
class X{
friend void T::foo();
};

In the friend declaration, the T:: part is a
nested-name-specifier (8
dcl.decl paragraph 4),
and T must be a class-name or a namespace-name
(5.1
expr.prim paragraph 7). However, according to
14.1
temp.param paragraph 3, it is only a
type-name. The fragment should be well-formed, and
instantiations of the template allowed as long as the actual template
argument is a class which provides a function member foo. As
a result of this defect, any usage of template parameters in nested
names is ill-formed, e.g., in the example of 14.6
temp.res paragraph 2.

Notes from 04/00 meeting:

The discussion at the meeting revealed a self-contradiction in the
current IS in the description of nested-name-specifiers.
According to the grammar in 5.1
expr.prim paragraph 7,
the components of a nested-name-specifier must be either
class-names or namespace-names, i.e., the constraint is
syntactic rather than semantic. On the other hand, 3.4.3
basic.lookup.qual paragraph 1 describes a semantic constraint: only
object, function, and enumerator names are ignored in the lookup for
the component, and the program is ill-formed if the lookup finds
anything other than a class-name or namespace-name. It
was generally agreed that the syntactic constraint should be
eliminated, i.e., that the grammar ought to be changed not to use
class-or-namespace-name.

A related point is the explicit prohibition of use of template
parameters in elaborated-type-specifiers in 7.1.5.3
dcl.type.elab paragraph 2. This rule was the result of an
explicit Committee decision and should not be unintentionally voided
by the resolution of this issue.

This resolution depends on the resolutions for issues
245 (to change the name lookup rules in
elaborated-type-specifiers to include all type-names)
and 283 (to categorize template
type-parameters as type-names).

Notes from 10/01 meeting:

There was some sentiment for going with simply identifier
in front of the "::", and stronger sentiment for going
with something with a more descriptive name if possible.
See also issue 180.

The name of a class or namespace member can be referred to after
the :: scope resolution operator (5.1
expr.prim) applied to a nested-name-specifier that
nominates its class or namespace. During the lookup for a name
preceding the :: scope resolution operator, object,
function, and enumerator names are ignored. If the name found
is not a class-name (clause 9
class) or
namespace-name (7.3.1
namespace.def)does
not designate a class or namespace, the program is
ill-formed. [...]

Notes from the April, 2005 meeting:

The 10/2004 resolution does not take into account the fact that
template type parameters do not designate class types in the context
of the template definition. Further drafting is required.

The name of a class or namespace member can be referred to after
the :: scope resolution operator (5.1
expr.prim) applied to a nested-name-specifier that
nominates its class or namespace. During the lookup for a name
preceding the :: scope resolution operator, object,
function, and enumerator names are ignored. If the name found
is not a class-name (clause 9
class) or
namespace-name (7.3.1
namespace.def)does
not designate a namespace or a class or dependent type, the program is
ill-formed. [...]

226.
Default template arguments for function templates

The prohibition of default template arguments for function
templates is a misbegotten remnant of the time where freestanding
functions were treated as second class citizens and required all
template arguments to be deduced from the function arguments
rather than specified.

The restriction seriously cramps programming style by unnecessarily making
freestanding functions different from member functions, thus making it harder
to write STL-style code.

Suggested resolution:

Replace

A default template-argument shall not be specified in a
function template declaration or a function template definition, nor
in the template-parameter-list of the definition of a member of
a class template.

by

A default template-argument shall not be specified in the
template-parameter-list of the definition of a member of a
class template.

The actual rules are as stated for arguments to class templates.

Notes from 10/00 meeting:

The core language working group was amenable to this change.
Questions arose, however, over the interaction between default
template arguments and template argument deduction: should it be
allowed or forbidden to specify a default argument for a deduced
parameter? If it is allowed, what is the meaning: should one or
the other have priority, or is it an error if the default and
deduced arguments are different?

Notes from the 10/01 meeting:

It was decided that default arguments should be allowed on friend
declarations only when the declaration is a definition. It was
also noted that it is not necessary to insist that if there
is a default argument for a given parameter all following parameters
have default arguments, because (unlike in the class case)
arguments can be deduced if they are not specified.

A default template-argument may be specified in a class
template declaration or a class template definition. A default
template-argument shall not be specified in a function
template declaration or a function template definition, nor in the
template-parameter-list of the definition of a member of a
class template.

with

A default template-argument may be specified in a template
declaration. A default
template-argument shall not be specified in the
template-parameter-lists of the definition of a member
of a class template that appears outside of the member's class.

A default template-argument shall not be specified in a friend template
declaration.

with

A default template-argument shall not be specified in a friend
class template declaration. If a friend function template declaration
specifies a default template-argument, that declaration shall be
a definition and shall be the only declaration of the function template
in the translation unit.

If a template-parameter has a default template-argument,
all subsequent template-parameters shall have a default
template-argument supplied.

with

If a template-parameter of a class template has a default
template-argument, all subsequent template-parameters
shall have a default template-argument supplied.
[Note: This is not a requirement for function templates
because template arguments might be deduced (14.8.2
temp.deduct).]

Template arguments can either be explicitly specified when
naming the function template specialization or be deduced
(14.8.2
temp.deduct) from the context, e.g. from the
function arguments in a call to the function template
specialization.

with

Template arguments can be explicitly specified when naming the
function template specialization, deduced from the context
(14.8.2
temp.deduct), e.g., deduced from the function
arguments in a call to the function template specialization), or
obtained from default template arguments.

The resulting substituted and adjusted function type is used as
the type of the function template for template argument
deduction. When all template arguments have been deduced, all
uses of template parameters in nondeduced contexts are replaced
with the corresponding deduced argument values. If the
substitution results in an invalid type, as described above,
type deduction fails.

with

The resulting substituted and adjusted function type is used as
the type of the function template for template argument
deduction. If a template argument has not been deduced, its
default template argument, if any, is used. [Example:

When all template arguments have been deduced or obtained from
default template arguments, all uses of template parameters in
nondeduced contexts are replaced with the corresponding deduced
or default argument values. If the substitution results in an
invalid type, as described above, type deduction fails.

The issue is whether the access to the default argument type BASE::PI
is checked before or after it is known that BASE is a base class of
the template. To some extent, one needs to develop the list of
template arguments (and therefore evaluate the default argument)
before one can instantiate the template, and
one does not know what base classes the template has until it has
been instantiated.

The convincing argument here is that if we had only the declaration
of the template (including the default argument), we would expect
it to be usable in exactly the same way as the version with the
definition. However, the special access needed is visible only
when the definition is available. So the above should be an error,
and information from the definition cannot affect the access of
the default arguments.

We decided that template parameter default arguments should
have their access checked in the context where they appear
without special access for the entity declared (i.e., they are
different than normal function default arguments). One reason:
we don't know the instance of the template when we need the
value. Second reason: compilers want to parse and throw away the
form of the template parameter default argument, not save it and
check it for each instantiation.

Class templates should be treated the same as function templates
in this regard. The base class information is in the same category
as friend declarations inside the class itself -- not available.
If the body were used one would need to instantiate it in order
to know whether one can name it.

Proposed resolution (October, 2004):

Add the following as a new paragraph following the last
paragraph of 11
class.access (but before the new
paragraph inserted by the resolution of issue 372, if adopted):

The names in a default template-argument (14.1
temp.param) have their access checked in the context in which
they appear rather than at any points of use of the default
template-argument. [Example:

This is an error because X is not a member template;
14.2
temp.names paragraph 5 says:

If a name prefixed by the keyword template is not the name
of a member template, the program is ill-formed.

In a way this makes perfect sense: X is found to be a template
using ordinary lookup even though p has a dependent type.
However, I think this makes the use of the template prefix
even harder to teach.

Was this intentionally outlawed?

Proposed Resolution (4/02):

Elide the first use of the word "member" in
14.2
temp.names paragraph 5 so that its first sentence reads:

If a name prefixed by the keyword template is not the name of a
member template, the program is ill-formed.

This is ill-formed according to the standard, since operator+ is
not a template-name.

Suggested resolution:

I think the right rule is

template-name:

identifieroperator-function-idconversion-function-id

John Spicer adds that there's some question about whether
conversion functions should be included, as they cannot have template
argument lists.

Notes from 4/02 meeting:

If the change is made as a syntax change, we'll need a semantic
restriction to avoid operator+<int> as a class.
Clark Nelson will work on a compromise proposal -- not the minimal
change to the syntax proposed, not the maximal change either.

Clark Nelson (April 2003):

The proposed solution (adding operator-function-id as an
alternative for template-name) would have a large impact on
the language described by the grammar. Specifically, for example,
operator+<int> would become a syntactically valid
class-name.

On the other hand, a change with (I believe) exactly the desired
effect on the language accepted, would be to modify
operator-function-id itself:

operator-function-id:

operatoroperatoroperatoroperator<template-argument-listopt>

(Steve Adamczyk: this change was already made by
issue 38 and is in TC1.)

After name lookup (3.4
basic.lookup)
finds that a name is a
template-name, if this name is followed by a
<, the < is always taken as the
beginning of a template-argument-list and never as a name
followed by the less-than operator.

This description seems to be adequate for names of class templates.
As far as I can tell, the only ambiguity it resolves is from something
that starts with new X <, in the
scope of a class template X. But as far as I can tell is
already inadequate for names of function templates, and is even worse
for operator function templates.

Probably < should always be interpreted as
introducing a template-argument-list if any member of the
overload set is a function template. After all, function pointers are
very rarely compared for ordering, and it's not clear what other rule
might be workable.

I'm inclined to propose the simplest rule possible for
operator-function-ids: if one is followed by
<, then what follows is interpreted as a
template-argument-list, unconditionally. Of course, if no
template for that operator has been declared, then there's an error.

Also, note that if the operator in question is < or <<, it is possible to run into a problem similar to the famous >> nested template argument list closing delimiter problem.
However, since in this case (at least) one of the < characters has a radically different interpretation than the other, and for other reasons as well, this is unlikely to be nearly as much of a practical problem as the >> problem.

Notes from April 2003 meeting:

We felt that the operator functions should not be special-cased.
They should be treated like any other name.

September 2003:

Clark Nelson has provided the changes in N1490=03-0073.

Notes from October 2003 meeting:

We reviewed Clark Nelson's N1490. Clark will revise it and
introduce a new syntax term for an identifier or
the name of an operator function.

Notes from the April, 2005 meeting:

The CWG suggested a new approach to resolving this issue: the
existing term template-id will be renamed
to class-template-id, the term template-id will be
defined to include operator functions with template arguments, and any
current uses of template-id (such as in the definition
of elaborated-type-specifier) where an operator function is not
appropriate will be changed to refer to class-template-id.

Proposed resolution (April, 2006):

As specified in document J16/05-0156 = WG21 N1896, except that:

In change 9 (3.4.5
basic.lookup.classref), omit the change from
“entire postfix-expression” to
“nested-name-specifier.”

In change a (3.4.3.1
class.qual paragraph 1,
third bullet), omit the change from “entire
postfix-expression” to “qualified-id.”

If a name prefixed by the keyword template is not the name of
a template, the program is ill-formed. [Note: the
keyword template may not be applied to non-template members
of class templates. —end note] Furthermore, names of
member templates shall not be prefixed by the
keyword template if the postfix-expression
or qualified-id does not appear in the scope of a template.
[Note: just as is the case with the typename prefix,
the template prefix is allowed in cases where it is not
strictly necessary; i.e., when the nested-name-specifier
or the expression on the left of the ->
or ., or the nested-name-specifier is not
dependent on a template-parameter, or the use does not
appear in the scope of a template. —end note]

A goto, break, return, or continue
statement can be used to transfer control out of a try block or
handler, but not into one.

However, 15.3
except.handle paragraph 14 mentions
only constructors and destructors for the prohibition:

If the handlers of a function-try-block contain a jump into the
body of a constructor or destructor, the program is ill-formed.

Is this paragraph simply reemphasizing the more general
restriction, or does it assume that such a jump would be permitted
for functions other than constructors or destructors? If the
former interpretation is correct, it is confusing and should be
either eliminated or turned into a note. If the latter
interpretation is accurate, 15
except paragraph 2
must be revised.

John Spicer:
I don't think the standard really addresses this case. There
is wording about access checking of things used as template arguments,
but that doesn't address accessing members of the template argument
type (or template) from within the template.

This example is similar, but does not use template template arguments.

This gets an error from most compilers, though the standard is probably
mute on this as well. An error makes sense -- if there is no error,
there is a hole in the access checking. (The special rule about
no access checks on template parameters is not a hole, because the access
is checked on the type passed in as an argument. But when you look
up something in the scope of a template parameter type, you need to
check the access to the member found.)

The logic in the template template parameter case should be similar:
anytime you look up something in a template-dependent class, the
member's access must be checked, because it could be different for
different template instances.

The question is: If C is derived from A as above, is it allowed to
access A::N before the classes opening '{'?

The main problem is that you need to access U's protected parts in C's
base-clause. This pattern is common when using policies, Andrei's Loki
library was bitten by it as he tried to make some parts of the policies
'protected' but some compilers rejected the code. They were right to
reject it, I think it's
11.4
class.friend/2 that applies here and prevents the code
above to be legal, although it addresses a different and reasonable
example. To me, it seems wrong to reject the code as it is perfectly
reasonable to write such stuff. The questions are:

Do you agree it's reasonable?

Is it a DR or is it a request for an extension?

Is DR 372 the right place to address it or shall it be a new DR?

Steve Adamczyk:
In other words, the point of the issue is over what range access derived
from base class specifiers is granted, and whether any part of that range
is the base specifier list itself, either the parts afterwards or the whole
base specifier list. (Clark Nelson confirms this is what he was
asking with the original question.)
Personally, I find it somewhat disturbing that access might
arrive incrementally; I'd prefer that the access happen all
at once, at the opening brace of the class.

Notes from October 2003 meeting:

We decided it makes sense to delay the access checking for the
base class specifiers until the opening brace of the class is
seen. In other words, the base specifiers will be checked using
the full access available for the class, and the order of the
base classes is not significant in that determination. The
implementors present all said they already had code to handle
accumulation of delayed access checks, because it is already
needed in other contexts.

For a template-argumentofthat is a class
type or a class template, the template definition
has no special access rights to the inaccessible members of the
template argument typetemplate-argument.
[Example:

The 10/2004 resolution is not sufficient to implement the CWG's
intent to allow these examples: clause 11
class.access
paragraph 1 grants protected access only to “members and
friends” of derived classes, not to their
base-specifiers. The resolution needs to be extended to say
either that access in base-specifiers is determined as if they
were members of the class being defined or that access is granted to
the class as an entity, including its base-specifiers. See
also issue 500, which touches on the same
issue and should be resolved in the same way.

protected; that is, its name can be used only by
members and friends of the class in which it is declared, and by
members and friends of classes derived from this classby
classes derived from that class, and by their friends (see
11.5
class.protected).

All access controls in clause 11
class.access affect the
ability to access a class member name from a particular scope. The
access control for names used in the definition of a class member that
appears outside of the member's class definition is done as if the
entire member definition appeared in the scope of the member's
class.For purposes of access control, the base-specifiers
of a class and the definitions of class members that appear outside of
the class definition are considered to be within the scope of that
class. In particular...

Change the example and commentary in 11
class.access
paragraphs 6-7 as indicated:

Here, all the uses of A::I are well-formed
because A::f and A::x are members of
class A and g is a friend of class A. This
implies, for example, that access checking on the first use
of A::I must be deferred until it is determined that this use
of A::I is as the return type of a member of
class A. Similarly, the use of A::B as a
base-specifier is well-formed because D is derived
from A, so access checking of base-specifiers must be
deferred until the entire base-specifier-list has been seen.
—end example]

Declaring a class to be a friend implies that the names of private
and protected members from the class granting friendship can be
accessed in declarations of members of the befriended
class. [Note: this means that access to private and protected
names is also granted to member functions of the friend class (as if
the functions were each friends) and to the static data member
definitions of the friend class. This also means that private and
protected type names from the class granting friendship can be used in
the base-clause of a nested class of the friend class. However,
the declarations of members of classes nested within the friend class
cannot access the names of private and protected members from the
class granting friendship. Also, because the base-clause of the
friend class is not part of its member declarations,
the base-clause of the friend class cannot access the names of
the private and protected members from the class granting
friendship. For example,

Declaring a class to be a friend implies that the names of private and
protected members from the class granting friendship can be accessed
in the base-specifiers and member declarations of the befriended
class. [Example:

For a template-argumentofthat is a class type
or a class template, the template definition has no special
access rights to the inaccessible members of the template
argument type.template-argument. [Example:

Like a member function, a friend function (11.4
class.friend)
defined within a nested class is in the lexical scope of that class;
it obeys the same rules for name binding as a static member function
of that class (9.4
class.static), but itand
has no special access rights to members of an enclosing class.

354.
Null as nontype template argument

The standard does not permit a null value to be used as a nontype template
argument for a nontype template parameter that is a pointer.

This code is accepted by EDG, Microsoft, Borland and Cfront, but rejected
by g++ and Sun:

template <int *p> struct A {};
A<(int*)0> ai;

I'm not sure this was ever explicitly considered by the committee.
Is there any reason to permit this kind of usage?

Jason Merrill:
I suppose it might be useful for a program to be able to express a
degenerate case using a null template argument. I think allowing it would
be harmless.

Notes from October 2004 meeting:

CWG decided that it would be desirable to allow null pointers
as nontype template arguments, even though they are not
representable in some current ABIs. There was some discussion
over whether to allow a bare 0 to be used with a pointer
nontype template parameter. The following case was decisive:

The current wording of 14.3
temp.arg paragraph 7
disambiguates the function call in favor of the int
version. If the null pointer conversion were allowed for pointer
nontype template parameters, this case would become ambiguous, so
it was decided to require a cast.

Proposed resolution (April, 2005):

In 14.3.2
temp.arg.nontype paragraph 1, insert the
following after the third bullet:

a constant expression that evaluates to a null pointer
value (4.10
conv.ptr); or

a constant expression that evaluates to a null member
pointer value (4.11
conv.mem); or

Add the indicated text to the note in the second bullet of
14.3.2
temp.arg.nontype paragraph 5:

[Note: In particular, neither the null pointer conversion
(4.10
conv.ptr) nor the derived-to-base conversion
(4.10
conv.ptr) are applied. Although 0 is a valid
template-argument for a non-type template-parameter of integral
type, it is not a valid template-argument for a non-type
template-parameter of pointer type. However, (int*)0 is a valid
template-argument for a non-type template-parameter of type
“pointer to int.” —end note]

Replace the normative wording of 14.4
temp.type paragraph 1 with the following:

Two template-ids refer to the same class or function
if

their template-names refer to the same template, and

their corresponding type template-arguments are the
same type, and

their corresponding non-type template-arguments of
integral or enumeration type have identical values, and

their corresponding non-type template-arguments of
pointer type refer to the same external object or function or are
both the null pointer value, and

their corresponding non-type template-arguments of
pointer-to-member type refer to the same class member or are both
the null member pointer value, and

their corresponding non-type template-argumentss for
template parameters of reference type refer to the same external
object or function, and

their corresponding template template-arguments refer
to the same template.

582.
Template conversion functions

The wholesale replacement of the phrase “template
function” by the resolution of issue 105 seems to have overlooked the similar phrase “template
conversion function.” This phrase appears a number of times in
13.3.3.1.2
over.ics.user paragraph 3, 14.5.2
temp.mem paragraphs 5-8, and 14.8.2
temp.deduct
paragraph 1. It should be systematically replaced in similar
fashion to the resolution of issue 105.

A specialization of a template conversion
functionconversion function template is not found by name
lookup. Instead, any template conversion
functionsconversion function templates visible in the
context of the use are considered.

Template argument deduction is done by comparing the return type of
the template conversion functionconversion function
template (call it P) with the type that is required as
the result of the conversion (call it A) as described in
14.8.2.5
temp.deduct.type.

329.
Evaluation of friends of templates

When a function is defined in a friend function declaration in a class
template, the function is defined at each instantiation of the class
template. The function is defined even if it is never used. The same
restrictions on multiple declarations and definitions which apply to
non-template function declarations and definitions also apply to these
implicit definitions. [Note: if the function definition is ill-formed
for a given specialization of the enclosing class template, the program
is ill-formed even if the function is never used. ]

This means that the following program is invalid, even without the call
of f(ai):

The EDG front end issues an error on this case even if f(ai)
is never
called. Of the compilers I tried (g++, Sun, Microsoft, Borland) we
are the only ones to issue such an error.

This issue came up because there is a library that either deliberately or
accidentally makes use of friend functions that are not valid for certain
instantiations.

The wording in the standard is the result of a deliberate decision made
long ago, but given the fact that most implementations do otherwise it
raises the issue of whether we did the right thing.

Upon further investigation, the current rule was adopted as the resolution
to issue 6.47 in my series of template issue papers. At the time the
issue was discussed (7/96) most compilers did evaluate such friends.
So it seems that a number of compilers have changed their behavior
since then.

Based on current practice, I think the standard should be changed to
evaluate such friends only when used.

When a function is defined
in a friend function declaration in a class template,
the function is defined at each instantiation of the class template.
The function is defined even if it is never used.
The same restrictions on multiple declarations and definitions
which apply to non-template function declarations and definitions
also apply to these implicit definitions.
[Note:
if the function definition is ill-formed for a given specialization of the
enclosing class template, the program is ill-formed even if the function
is never used.
]

to:

When a function is defined
in a friend function declaration in a class template,
the function is instantiated when the function is used.
The same restrictions on multiple declarations and definitions
that apply to non-template function declarations and definitions
also apply to these implicit definitions.

A friend function declaration that is not a template declaration and
in which the name of the friend is an unqualified template-id shall
refer to a specialization of a function template declared in the
nearest enclosing namespace scope. [Example:

A friend function declaration that is not a template declaration and
in which the name of the friend is an unqualified template-id shall
refer to a specialization of a function template declared in the
nearest enclosing namespace scope. [Example:

517.
Partial specialization following explicit instantiation

If a template is partially specialized then that partial
specialization shall be declared before the first use of that partial
specialization that would cause an implicit instantiation to take
place, in every translation unit in which such a use occurs; no
diagnostic is required.

This leaves the impression that an explicit instantiation of the
primary template may precede the declaration of an applicable partial
specialization. Is the following example well-formed?

If a template is partially specialized then that partial
specialization shall be declared before the first use of that partial
specialization that would cause an implicit instantiation to take
place, in every translation unit in which such a use occurs; no
diagnostic is required.

with:

A partial specialization shall be declared before the first use of a
class template specialization that would make use of the partial
specialization as the result of an implicit or explicit instantiation
in every translation unit in which such a use occurs; no diagnostic is
required.

214.
Partial ordering of function templates is underspecified

In 14.5.5.2
temp.func.order, partial ordering is explained in
terms of template argument deduction. However, the exact procedure for
doing so is not specified. A number of details are missing, they are
explained as sub-issues below.

14.5.5.2
temp.func.order paragraph 2 refers to
14.8.2
temp.deduct for argument deduction. This is the
wrong reference; it explains how explicit arguments are processed
(paragraph 2) and how function parameter types are adjusted (paragraph
3). Neither of these steps is meaningful in the context of partial
ordering. Next in deduction follows one of the steps in 14.8.2.1
temp.deduct.call, 14.8.2.2
temp.deduct.funcaddr, 14.8.2.3
temp.deduct.conv, or 14.8.2.5
temp.deduct.type. The standard
does not specify which of these contexts apply to partial
ordering.

Because 14.8.2.1
temp.deduct.call and 14.8.2.3
temp.deduct.conv both start with actual function parameters, it is
meaningful to assume that partial ordering uses 14.8.2.5
temp.deduct.type, which only requires types. With that assumption,
the example in 14.5.5.2
temp.func.order paragraph 5 becomes
incorrect, considering the two templates

Here, #2 is at least as specialized as #1: With a synthetic type
U, #2 becomes g(U&); argument deduction against
#1 succeeds with T=U&. However, #1 is not at least as
specialized as #2: Deducing g(U) against g(T&)
fails. Therefore, the second template is more specialized than the
first, and the call g(x) is not ambiguous.

According to John Spicer, the intent of the partial ordering was
that it uses deduction as in a function call (14.8.2.1
temp.deduct.call), which is indicated by the mentioning of "exact
match" in 14.5.5.2
temp.func.order paragraph 4. If that is
indeed the intent, it should be specified how values are obtained for
the step in 14.8.2.1
temp.deduct.call paragraph 1, where the
types of the arguments are determined. Also, since 14.8.2.1
temp.deduct.call paragraph 2 drops references from the parameter
type, symmetrically, references should be dropped from the argument
type (which is done in 5
expr paragraph 2, for a true
function call).

14.5.5.2
temp.func.order paragraph 4 requires an "exact
match" for the "deduced parameter types". It is not clear whether this
refers to the template parameters, or the parameters of the template
function. Considering the example

Here, #3 is clearly at least as specialized as #1. To determine
whether #1 is at least as specialized as #3, a unique type U
is synthesized, and deduction of g<U>(U) is performed
against #3. Following the rules in 14.8.2.1
temp.deduct.call,
deduction succeeds with T=U. Since the template argument is
U, and the deduced template parameter is also U, we
have an exact match between the template parameters. Even though the
conversion from U to U const & is an exact
match, it is not clear whether the added qualification should be taken
into account, as it is in other places.

Issue 200 covers a related issue,
illustrated by the following example:

A template is more specialized than another if, and
only if, it is at least as specialized as the other
template and that template is not at least as
specialized as the first.

To paraphrase, given two templates A & B, if A's template
parameters can be deduced by B, but B's cannot be deduced by
A, then A is more specialized than B. Deduction is done as
if for a function call. In particular, except for conversion
operators, the return type is not involved in deduction.
This leads to the following templates and use being
unordered. (This example is culled from G++ bug report 4672
http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view&pr=4672)

It looks like #2 is more specialized that #1, but
14.5.5.2
temp.func.order does not make it so, as neither template can
deduce 'T' from the other template's function parameters.

Possible Resolutions:

There are several possible solutions, however through
experimentation I have discounted two of them.

Option 1:

When deducing function ordering, if the return type of one of
the templates uses a template parameter, then return types
should be used for deduction. This, unfortunately, makes
existing well formed programs ill formed. For example

Here, neither #1 nor #2 can deduce the other, as the return
types fail to match. Considering only the function parameters
gives #2 more specialized than #1, and hence makes the call
#3 well formed.

Option 2:

As option 1, but only consider the return type when deducing
the template whose return type involves template parameters.
This has the same flaw as option 1, and that example is
similarly ill formed, as #1's return type 'X<T,0>' fails to
match 'int' so #1 cannot deduce #2. In the converse direction,
return types are not considered, but the function parameters
fail to deduce.

Option 3:

It is observed that the original example is only callable
with a template-id-expr to supply a value for the first,
undeducible, parameter. If that parameter were deducible it
would also appear within at least one of the function
parameters. We can alter paragraph 4 of [temp.func.order]
to indicate that it is not necessary to deduce the parameters
which are provided explicitly, when the call has the form
of a template-id-expr. This is a safe extension as it only
serves to make ill formed programs well formed. It is also
in line with the concept that deduction for function
specialization order should proceed in a similar manner to
function calling, in that explicitly provided parameter
values are taken into consideration.

Suggested resolution:

Insert after the first sentence of paragraph 4 in

Should any template parameters remain undeduced, and
the function call be of the form of a template-id-expr,
those template parameters provided in the
template-id-expr may be arbitrarily synthesized prior
to determining whether the deduced arguments generate
a valid function type.

Partial ordering selects which of two function templates is more
specialized than the other by transforming each template in turn
(see next paragraph) and performing template argument deduction
using the function parameter types, or in the case of a conversion
function the return type. The deduction process determines whether
one of the templates is more specialized than the other. If so, the
more specialized template is the one chosen by the partial ordering
process.

To produce the transformed template, for each type, non-type, or template
template parameter synthesize a unique type, value, or class template
respectively and substitute it for each occurrence of that parameter
in the function type of the template.

Using the transformed function template's function parameter list, or
in the case of a conversion function its transformed return type,
perform type deduction against the function parameter list (or return
type) of the other function. The mechanism for performing these
deductions is given in 14.8.2.x.

Remove the text of 14.5.5.2
temp.func.order paragraph 5 but
retain the example. The removed text is:

A template is more specialized than another if, and only if, it is
at least as specialized as the other template and that template is not
at least as specialized as the first.

Insert the following section before 14.8.2.5
(Note that this would either be a new 14.8.2.4, or would be given
a number like 14.8.2.3a. If neither of these is possible from
a troff point of view, this could be made 14.8.2.5.
)

Template argument deduction is done by comparing certain types associated with
the two function templates being compared.

Two sets of types are used to determine the partial ordering. For each of
the templates involved there is the original function type and the
transformed function type. [Note:
The creation of the transformed type is
described in 14.5.5.2
temp.func.order.]
The deduction process uses the
transformed type as the argument template and the original type of the
other template as the parameter template. This process is done twice
for each type involved in the partial ordering comparison: once using
the transformed template-1 as the argument template and template-2 as
the parameter template and again using the transformed template-2 as
the argument template and template-1 as the parameter template.

The types used to determine the ordering depend on the context in which
the partial ordering is

In the context of a function call, the function parameter types
are used.

In the context of a call to a conversion operator, the return types of
the conversion function templates are used.

In other contexts (14.5.5.2
temp.func.order),
the function template's function type is used.

Each type from the parameter template and the corresponding type from the
argument template are used as the types of P and A.

Before the partial ordering is done, certain transformations are performed
on the types used for partial ordering:

If P is a reference type, P is replaced by the type
referred to.

If A is a reference type, A is replaced by the type
referred to.

If both P and A were reference types (before being replaced
with the type referred to above), determine which
of the two types (if any) is more cv-qualified than the other; otherwise
the types are considered to be equally cv-qualified for partial ordering
purposes. The result of this determination will be used below.

Remove any top-level cv-qualifiers:

If P is a cv-qualified type, P is replaced by the
cv-unqualified version of P.

If A is a cv-qualified type, A is replaced by the
cv-unqualified version of A.

Using the resulting types P and A the deduction is then
done as described in (14.8.2.5
temp.deduct.type).
If deduction succeeds for a given type,
the type from the argument template is considered to be at least as specialized
as the type from the parameter template.

If, for a given type, deduction succeeds in both directions (i.e., the
types are identical after the transformations above) if the type from
the argument template is more cv-qualified than the type from the
parameter template (as described above) that type is considered to be
more specialized than the other. If neither type is more cv-qualified
than the other then neither type is more specialized than the other.

If for each type being considered a given template is at least as specialized
for all types and more specialized for some set of types and the
other template is not more specialized for any types or is not
at least as specialized for any types, then
the given template is more specialized than the other template.
Otherwise, neither template
is more specialized than the other.

In most cases, all template parameters must have values in order for
deduction to succeed, but for partial ordering purposes a template
parameter may remain without a value provided it is not used in the
types being used for partial ordering. [Note:
A template parameter used
in a non-deduced context is considered used.]

180.
typename and elaborated types

Mike Miller:
A question about typename came up in the discussion of
issue 68 that is somewhat relevant
to the idea of omitting
typename in contexts where it is clear that a type is
required: consider something like

template <class T>
class X {
friend class T::nested;
};

Is typename required here? If so, where would it go? (The
grammar doesn't seem to allow it anywhere in an
elaborated-type-specifier that has a class-key.)

Bill Gibbons:
The class applies to the last identifier in the qualified name,
since all the previous names must be classes or namespaces. Since the
name is specified to be a class it does not need typename.
[However,] it looks like
14.6
temp.res
paragraph 3 requires
typename and the following paragraphs
do not exempt this case. This is not what we agreed on.

The keyword typename is not permitted in a
base-specifier or in a mem-initializer; in these
contexts a qualified-name that depends on a
template-parameter (14.6.2
temp.dep) is
implicitly assumed to be a type name.

to

A qualified name used as the name in a mem-initializer-id,
a base-specifier, or an elaborated-type-specifier
(in the class-key and enum forms) is implicitly
assumed to name a type, without the use of the typename
keyword. [Note: the typename keyword is not
permitted by the syntax of these constructs.]

(The expected resolution for issue 254 will remove the typename forms from the
grammar for elaborated-type-specifier. If that
resolution is adopted, the parenthetical phrase "(in the
class-key and enum forms)" in the preceding
wording should be removed because those will be the only forms
of elaborated-type-specifier.)

This has been consolidated with the edits for some other
issues. See N1376=02-0034.

382.
Allow typename outside of templates

P. J. Plauger, among others, has noted that typename is hard
to use, because in a given context it's either required or forbidden,
and it's often hard to tell which.
It would make life easier for programmers if typename
could be allowed in places where it is not required, e.g., outside
of templates.

Notes from the April 2003 meeting:

There was unanimity on relaxing this requirement on typename.
The question was how much to relax it. Everyone agreed on allowing
it on all qualified names, which is an easy fix (no syntax change
required). But should it be allowed other places? P.J. Plauger
said he'd like to see it allowed anywhere a type name is allowed,
and that it could actually be a decades-late assist for the infamous
"the ice is thin here" typedef problem noted in K&R I.

A qualified-id that refers to a type and in which
the nested-name-specifier
depends on a template-parameter (14.6.2
temp.dep)
shall be prefixed by the keyword
typename to indicate that
the qualified-id denotes a type, forming an
elaborated-type-specifier (7.1.5.3
dcl.type.elab).

With:

The keyword typename can only be applied to a qualified-id. A
qualified-id that refers to a type and in which
the nested-name-specifier
depends on a template-parameter (14.6.2
temp.dep)
shall be prefixed by the keyword
typename to indicate that the qualified-id
denotes a type, forming
an elaborated-type-specifier (7.1.5.3
dcl.type.elab).
If a qualified-id which
has been prefixed by the keyword typename does not
denote a type the program is ill-formed.
[ Note: The keyword is only required on a qualified-id within a
template declaration or definition in which the
nested-name-specifier depends on a template-parameter. ]

The keyword typename shall only be used in template
declarations and definitions, including in the return type of a function template or
member function template, in the return type for the definition of a member function
of a class template or of a class nested within a class template, and in the
type-specifier for the definition of a static member of a class template or
of a class nested within a class template. The keyword typename shall be
applied only to qualified names, but those names need not be dependent. The keyword
typename shall be used only in contexts in which dependent names
can be used. This includes template declarations and definitions but excludes explicit
specialization declarations and explicit instantiation declarations. The keyword
typename
is not permitted in a base-specifier or in a mem-initializer; in these
contexts a qualified-id that depends on a template-parameter (temp.dep) is
implicitly assumed to be a type name.

Note: the claim here that a qualified name preceded by
typename forms an elaborated type specifier conflicts with
the changes made in issue 254 (see
N1376=02-0034), which introduces typename-specifier.

Notes from October 2003 meeting:

We considered whether typename should be allowed in more
places, and decided we only wanted to allow it in qualified
names (for now at least).

AWhen aqualified-idthat refers to a
type and in which the nested-name-specifier depends on a
template-parameter (14.6.2
temp.dep) is
intended to refer to a type, it shall be prefixed by the
keyword typenameto indicate that the qualified-id
denotes a type, forming a typename-specifier. If
the qualified-id in a typename-specifier does not
denote a type, the program is ill-formed.

The keyword typename shall only be used in template
declarations and definitions, including in the return type of a
function template or member function template, in the return type
for the definition of a member function of a class template or of
a class nested within a class template, and in the
type-specifier for the definition of a static member of a
class template or of a class nested within a class template. The
keyword typename shall be applied only to qualified
names, but those names need not be dependent. The keyword
typename shall be used only in contexts in which
dependent names can be used. This includes template declarations
and definitions but excludes explicit specialization declarations
and explicit instantiation declarations. A qualified name
used as the name in a mem-initializer-id, a
base-specifier, or an elaborated-type-specifier is
implicitly assumed to name a type, without the use of the
typename keyword. [Note: the typename
keyword is not permitted by the syntax of these
constructs. —end note]

409.
Obsolete paragraph missed by changes for issue 224

Paragraph 6 of 14.6
temp.res is obsolete as result of
issue 224, and needs to be revised.

Within the definition of a class template or within the definition of
a member of a class template, the keyword typename is not required
when referring to the unqualified name of a previously declared member
of the class template that declares a type. The keyword typename
shall always be specified when the member is referred to using a qual-
ified name, even if the qualifier is simply the class template name.
[Example:

Within the definition of a class template or within the definition of
a member of a class template, the keyword typename is not
required when referring to the unqualified name of a previously
declared member of the class template that declares a type.
The keyword typename shall always be specified when the
member is referred to using a qualified name, even if the qualifier is
simply the class template name. [Example:

559.
Editing error in issue 382 resolution

Part of the resolution for issue 224 was
the addition of the phrase “but does not refer to a member of
the current instantiation” to 14.6
temp.res
paragraph 3. When the resolution of issue 382
was added to the current working draft, however, that phrase was
inadvertently removed. Equivalent phrasing should be restored.

Proposed resolution (April, 2006):

Replace the first sentence of 14.6
temp.res paragraph 3
with the following text:

When a qualified-id is intended to refer to a type that is not
a member of the current instantiation (14.6.2.1
temp.dep.type) and its nested-name-specifier depends on a
template-parameter (14.6.2
temp.dep), it shall be
prefixed by the keyword typename, forming
a typename-specifier.

According to one
analysis, because the expression A::foo_ on line #1
is non-dependent, it must be analyzed in the definition
context. It that context, it violates the restrictions of
9.2
class.mem paragraph 10 on how the name of a
nonstatic data member of a class can be used and thus should be
treated as an error.

On the other hand, the description of the transformation of
an id-expression into a class member access expression
(9.3.1
class.mfct.nonstatic paragraph 3) does not have any special
treatment of templates; when C<int>::foo() is
instantiated, the reference to A::foo_ turns out to be to a
base class member and is thus transformed into
(*this).A::foo_ and is thus not an error.

When an id-expression (5.1
expr.prim) that is not
part of a class member access syntax (5.2.5
expr.ref)
and not used to form a pointer to member (5.3.1
expr.unary.op) is used in the body of a non-static member function
of class X or used in the mem-initializer for a
constructor of class X, if name lookup (3.4.1
basic.lookup.unqual) resolves the name in the id-expression to a
non-static non-type member of class X or of a base class
of Xsome class C,
the id-expression is transformed into a class member access
expression (5.2.5
expr.ref) using (*this)
(9.3.2
class.this) as the
postfix-expression to the left of the .
operator. [Note: If C is not X or a base
class of X, the class member access expression is ill-formed.
—end note]The member name then refers to the
member of the object for which the function is called. Similarly
during name lookup...

524.
Can function-notation calls to operator functions be dependent?

The description of dependent function calls in
14.6.2
temp.dep paragraph 1 applies only to
identifiers in postfix-notation function calls and to operator
notation calls for operator functions:

In an expression of the form:

postfix-expression(expression-listopt)

where the postfix-expression is an identifier,
the identifier denotes a dependent name if and only if
any of the expressions in the expression-list is a
type-dependent expression (14.6.2.2
temp.dep.expr). If an
operand of an operator is a type-dependent expression, the operator
also denotes a dependent name.

It would appear from the related passage in
14.6.4.2
temp.dep.candidate paragraph 1 that the description of
postfix-notation function calls should apply to all
unqualified-ids that are not template-ids,
including operator-function-ids, not just to
identifiers:

For a function call that depends on a template parameter, if the
function name is an unqualified-id but not
a template-id, the candidate functions are found...

where the postfix-expression is an identifierunqualified-id but not a template-id,
the identifierunqualified-id denotes
a dependent name if and only if any of the expressions in
the expression-list is a type-dependent expression
(14.6.2.2
temp.dep.expr)...

For a function call that depends on a template parameter, if the
function name is an unqualified-id but not
a template-id, or if the function is called using operator
notation, the candidate functions are found using the
usual lookup rules (3.4.1
basic.lookup.unqual, 3.4.2
basic.lookup.argdep) except that...

224.
Definition of dependent names

The definition of when a type is dependent, given in
14.6.2.1
temp.dep.type, is essentially syntactic: if the
reference is a qualified-id and one of the class-names
in the nested-name-specifier is dependent, the type is
dependent. This approach leads to surprising results:

The decision on whether a name is dependent or non-dependent should
be based on lookup, not on the form of the name: if the name can be
looked up in the definition context and cannot be anything else as the
result of specialization, the name should be non-dependent.

In the definition of a class template, a nested class of a class
template, a member of a class template, or a member of a nested class
of a class template, a name refers to the current
instantiation if it is

the injected-class-name (clause 9
class) of the
class template or nested class,

in the definition of a primary class template, the name of the
class template followed by the template argument list of the
primary template (as described below) enclosed in <>,

in the definition of a nested class of a class template,
the name of the nested class referenced as a member of the
current instantiation, or

in the definition of a partial specialization, the name of
the class template followed by the template argument list of
the partial specialization enclosed in <>.

The template argument list of a primary template is a
template argument list in which the nth template
argument has the value of the nth template
parameter of the class template.

A template argument that is equivalent to a template
parameter (i.e., has the same constant value or the same type
as the template parameter) can be used in place of that
template parameter in a reference to the current
instantiation. In the case of a nontype template argument,
the argument must have been given the value of the
template parameter and not an expression involving the
template parameter.

An unqualified name that, when looked up, refers to a member
of a class template. [Note: This can only occur when
looking up a name in a scope enclosed by the definition of a
class template.]

A qualified-id in which the nested-name-specifier
refers to the current instantiation.

[Example:

template <class T> class A {
static const int i = 5;
int n1[i]; // i refers to a member of the current instantiation
int n2[A::i]; // A::i refers to a member of the current instantiation
int n3[A<T>::i]; // A<T>::i refers to a member of the current instantiation
int f();
};
template <class T> int A<T>::f()
{
return i; // i refers to a member of the current instantiation
}

—end example]

A name is a member of an unknown specialization if the
name is a qualified-id in which the
nested-name-specifier names a dependent type that is not
the current instantiation.

A type is dependent if it is

a template parameter,

a member of an unknown specialization,

a nested class that is a member of the current
instantiation,

a cv-qualified type where the cv-unqualified type is dependent,

a compound type constructed from any dependent type,

an array type constructed from any dependent type or whose
size is specified by a constant expression that is value-dependent, or

a template-id in which either the template name is a
template parameter or any of the template arguments is a dependent
type or an expression that is type-dependent or value-dependent.

[Note: Because typedefs to not introduce new types, but
instead simply refer to other types, a name that refers to a
typedef that is a member of the current instantiation is dependent
only if the type referred to is dependent.]

A class member access expression (5.2.5
expr.ref) is
type-dependent if the type of the referenced member is dependent.
[Note: In an expression of the form x.y or
xp->y the type of the expression is usually the type
of the member y of the class of x (or the class
pointed to by xp). However, if x or xp
refers to a dependent type that is not the current instantiation,
the type of y is always dependent. If x or
xp refers to a non-dependent type or refers to the
current instantiation, the type of y is the type of the
class member access expression.]

When the name of a member template specialization appears after
. or -> in a postfix-expression, or
after a nested-name-specifier in a qualified-id,
and the postfix-expression or qualified-id explicitly
depends on a template-parameter (14.6.2
temp.dep),
the member template name must be prefixed by the keyword
template. Otherwise the name is assumed to name a
non-template.

with

When the name of a member template specialization appears after
. or -> in a postfix-expression, or
after a nested-name-specifier in a qualified-id,
and the postfix-expression or qualified-id explicitly
depends on a template-parameter (14.6.2
temp.dep)
but does not refer to a member of the current instantiation
(14.6.2.1
temp.dep.type), the member template name must
be prefixed by the keyword template. Otherwise the name
is assumed to name a non-template.

In 14.6.1
temp.local paragraph 2, remove the
following text, which was added for issue 108.
The updated definition of dependent name now addresses this case.

Within the scope of a class template, when the unqualified name of
a nested class of the class template is referred to, it is
equivalent to the name of the nested class qualified by the name
of the enclosing class template. [Example:

447.
Is offsetof type-dependent?

As far as I can tell, the standard doesn't say whether "offsetof(...)"
is type-dependent. In the abstract, it shouldn't be -- an "offsetof"
expression is always of type "size_t". But the standard doesn't say to
what the definition of the macro is, so I don't think one can deduce
that it will always be considered non-dependent by a conforming
compiler.

John Spicer:
(1) I agree that you can't know if offsetof is dependent because you
don't know what it expands to.
(2) In principle, offsetof should be like sizeof -- it is value-dependent
if its argument is type-dependent.

Mark Mitchell:
I think we should say that:
(a) offsetof is not type-dependent, and
(b) offsetof is value dependent iff the first argument is type-dependent

Everyone is using slightly different builtins to implement this
functionality, and I don't think that there's any guarantee that they're all
behaving the same here.

Notes from the March 2004 meeting:

Note that any such requirement would be in the library section,
not core.

Proposed resolution (October, 2004):

At the end of 14.6.2.2
temp.dep.expr paragraph
4, add after the list that ends with throwassignment-expression:

The macro offsetof accepts a restricted set of
type arguments in this International Standard. If
type is not a POD structure or a POD union the results are
undefined. The result of applying the offsetof macro to
a field that is a static data member or a function member is
undefined.

with

The macro offsetof(type,member-designator) accepts a restricted set of
type arguments in this International Standard. If
type is not a POD structure or a POD union (clause
9
class), the results are undefined. The
expression offsetof(type,member-designator) is never type-dependent
(14.6.2.2
temp.dep.expr) and it is value-dependent
(14.6.2.3
temp.dep.constexpr) if and only if type is
dependent. The result of applying the offsetof macro to
a field that is a static data member or a function member is
undefined.

[Note: the original wording shown here reflects the
resolutions of library issues 306 and 449.]

Since 14.6.4.2
temp.dep.candidate
says that
only Koenig lookup is done from the instantiation context, and since
3.4.2
basic.lookup.argdep
says that
fundamental types have no associated namespaces, either the example is
incorrect (and f(int) will never be called) or the
specification in 14.6.4.2
temp.dep.candidate
is incorrect.

Notes from 04/00 meeting:

The core working group agreed that the example as written is
incorrect and should be reformulated to use a class type instead of a
fundamental type. It was also decided to open a new issue dealing
more generally with Koenig lookup and fundamental types.

259.
Restrictions on explicit specialization and instantiation

No program shall explicitly instantiate any template more than once,
both explicitly instantiate and explicitly specialize a template, or
specialize a template more than once for a given set of
template-arguments.

This rule has an impact on library issue 120. Library authors
would like to have the freedom to specialize (or not) various
library functions without having to document their choices, while
users need the flexibility to explicitly instantiate library
functions in certain translation units.

If this rule could be slightly weakened, it would reduce the need
for constraining either the library author or the programmer. For
instance, the rule might be recast to say that if a specialization is
followed by an explicit instantiation in the same translation unit,
the explicit instantiation is ignored. A specialization and an
explicit instantiation of the same template in two different
translation units would still be an error, no diagnostic required.

No program shall explicitly instantiate any template more
than once, both explicitly instantiate and explicitly
specialize a template, or specialize a template more than
once for a given set of template-arguments.

by

For a given template and a given set of template-arguments,

an explicit instantiation shall appear at most once in a
program,

an explicit specialization shall be defined at most once
according to 3.2
basic.def.odr in a program, and

both an explicit instantiation and a declaration of an
explicit specialization shall not appear in a program unless
the explicit instantiation follows a declaration of the explicit
specialization.

The definition of a non-exported function template, a non-exported
member function template, or a non-exported member function or
static data member of a class template shall be present in every
translation unit in which it is explicitly instantiated.

by

For a given set of template parameters, if an explicit
instantiation of a template appears after a declaration of
an explicit specialization for that template, the explicit
instantiation has no effect. Otherwise, the definition of a
non-exported function template, a non-exported member
function template, or a non-exported member function or static
data member of a class template shall be present in every
translation unit in which it is explicitly instantiated.

A template is implicitly instantiated because of a "pointer conversion"
on an argument. This was intended to include related-class conversions,
but it also inadvertently includes conversions to void*, null
pointer conversions, cv-qualification conversions and the identity conversion.

It is not clear whether a reinterpret_cast of a pointer should
cause implicit instantiation.

Proposed resolution (10/01): Replace
14.7.1
temp.inst
paragraph 4, up to the
example, with the following:

A class template specialization is implicitly instantiated if the
class type is used in a context that requires a completely-defined
object type or if the completeness of the class type might affect the
semantics of the program. [Note: in particular, if the
semantics of an expression depend on the member or base class lists
of a class template specialization, the class template specialization
is implicitly generated. For instance, deleting a pointer to class
type depends on whether or not the class declares a destructor, and
conversion between pointer to class types depends on the inheritance
relationship between the two classes involved. ]

This version differs from the previous version is its use of the
word "might" in the first sentence.

237.
Explicit instantiation and base class members

The explicit instantiation of a class template specialization
implies the instantiation of all of its members not previously
explicitly specialized in the translation unit containing the
explicit instantiation.

Is "member" intended to mean "non-inherited member?"
If yes, maybe it should be clarified since
10
class.derived paragraph 1 says,

Unless redefined in the derived class, members of a
base class are also considered to be members of the derived
class.

470.
Instantiation of members of an explicitly-instantiated class template

The explicit instantiation of a class template specialization
implies the instantiation of all of its members not previously
explicitly specialized in the translation unit containing the
explicit instantiation.

It's not clear whether this “implied”
instantiation is implicit or explicit instantiation. It makes a
difference in cases like the following:

If the instantiation of foo<int>::bar implied
by #1 is implicit, the explicit instantiation in
#2 is well-formed. Otherwise, #2 violates the
requirement in 14.7
temp.spec that

No program shall explicitly instantiate any template more than
once ... for a given set of template-arguments.

It's also unclear whether the implied instantiation applies
only to direct members of the class template or to inherited
members, as well.

John Spicer: I have always interpreted this as meaning
only the members declared in the class, not those inherited from
other classes. This is what EDG does, and appears to be what
g++, Microsoft and Sun do, too. I also think this is the correct
thing for the Standard to require. If I were to derive a class
from a class in the standard library, an explicit instantiation
of my class should not cause the explicit instantiation of things
in the standard library (because the library might provide such
explicit instantiations, thus causing my program to run afoul of
the "can't instantiate more than once" rule).

The explicit instantiation of a class template specialization
implies the instantiation of allalso explicitly
instantiates each of its members not(not including
members inherited from base classes) whose definition is visible at
the point of instantiation and that has not been previously
explicitly specialized in the translation unit containing the explicit
instantiation.

An explicit specialization shall be declared in the namespace
of which the template is a member, or, for member templates, in the namespace
of which the enclosing class or enclosing class template is a member. An
explicit specialization of a member function, member class or static data
member of a class template shall be declared in the namespace of which
the class template is a member. ...

claiming that the specialization above is not "in the namespace of which
the enclosing class ... is a member". Elsewhere, declarations are sometimes
required to be "at" or "in" "namespace scope", which is not what it says
here. Paragraph 17 says:

A member or a member template may be nested within many enclosing
class templates. If the declaration of an explicit specialization for such
a member appears in namespace scope, the member declaration shall be preceded
by a template<> for each
enclosing class template that is explicitly
specialized.

The qualification "if the declaration ... appears in namespace scope",
implies that it might appear elsewhere. The only other place I can think
of for a member specialization is in class scope.

Was it the intent of the committee to forbid the construction above?
(Note that A itself is not a template.) If so, why?

If the declaration of an explicit specialization for such a member
appears in namespace scope...

with

In an explicit specialization for such a member...

Notes from 04/00 meeting:

This issue was kept in "review" status for two major reasons:

It's not clear that a change is actually needed. All uses of the
phrase "in the namespace" in the IS mean "directly in the namespace,"
not in a scope nested within the namespace.

There was substantial sentiment for actually adding support for
in-class specializations at a future time, and it might be perceived
as a reversal of direction to pass a change aimed at reinforcing the
absence of the feature, only to turn around afterward and add it.

Notes from 10/00 meeting:

The core working group felt that the value of additional
clarity here outweighs the potential disadvantages that were noted
at the preceding meeting.

The question here is whether unqualified names made visible by a
using-directive can be used as the declarator in an explicit
instantiation or explicit specialization.

Note that this question is already answered for qualified names in
8.3
dcl.meaning paragraph 1. In a qualified name such as
N::f, f must be a member of class or namespace
N, not a name made visible in N by a
using-directive (or a using-declaration, for that
matter).

The standard does not, as far as I can tell, specify the behavior of
these cases one way or another.

My opinion is that names from using-directives should not be
considered when looking up the name in an unqualified declarator in an
explicit specialization or explicit instantiation. In such cases, it
is reasonable to insist that the programmer know exactly which
template is being specialized or instantiated, and that a qualified
name must be used if the template is a member of a namespace.

As the example illustrates, allowing names from
using-directives to be used would also have the affect of
making ambiguous otherwise valid instantiation and specialization
directives.

Furthermore, permitting names from using-directives would
require an additional rule to prohibit the explicit instantiation of
an entity in one namespace from being done in another (non-enclosing)
namespace (as in the instantiation of f in namespace
M in the example).

Mike Miller: I believe the explicit specialization case
is already covered by 7.3.1.2
namespace.memdef paragraph 2,
which requires using a qualified name to define a namespace member
outside its namespace.

John Spicer: 7.3.1.2
namespace.memdef deals with
namespace members. An explicit specialization directive deals with
something that is a specialization of a namespace member. I don't
think the rules in 7.3.1.2
namespace.memdef could be taken to
apply to specializations unless the standard said so explicitly.

Proposed resolution (suggested 04/01, proposed 10/01):

(The first change below will need to be revised in accordance
with the resolution of issue 284 to
add a cross-reference to the text dealing with class names.)

An explicit instantiation shall appear in an enclosing namespace
of its template. If the name declared in the explicit
instantiation is an unqualified name, the explicit instantiation
shall appear in the namespace where its template is declared.
[Note: Regarding qualified names in declarators, see
8.3
dcl.meaning.]

If the declaration is not a definition, the specialization may be
defined later in the namespace in which the explicit specialization
was declared, or in a namespace that encloses the one in which the
explicit specialization was declared.

to

If the declaration is not a definition, the specialization may be
defined later (7.3.1.2
namespace.memdef).

The explicit specialization of member A<int>::B<double>::mf1
is ill-formed. The class template A<int>::B
is explicitly specialized and
contains no members, so any implicit specialization (such as
A<int>::B<double>) would also contain no members.

The wording in the standard that normally would
cover this (third sub-bullet in 14.8.2
temp.deduct
paragraph 2) says:

Attempting to use a type in the qualifier portion of a
qualified name that names a type when that type does
not contain the specified member, or if the specified
member is not a type where a type is required.

(emphasis mine). If the phrase "that names a
type" applies to "a qualified name," then the example
is invalid. If it applies to "the qualifier portion,"
then it is valid (because the second candidate is simply
discarded).

I suspect we want this example to work. Either way,
I believe the sub-bullet deserves clarification.

Notes from April 2003 meeting:

We agreed that the example should be valid. The phrase "that
names a type" applies to "the qualifier portion."

Attempting to use a type in the qualifier portion of a qualified name
that names a type when that type does not contain the
specified member, or if the specified member is not a type
where a type is required.

With

Attempting to use a type
in a nested-name-specifier of a qualified-id
when that type does not contain the specified member, or

the specified member is not a type where a type is required, or

the specified member is not a template where a template is required,
or

Here, the return type of f is deduced as
int[5], which is invalid according to 8.3.5
dcl.fct paragraph 6. The outcome of this example, then,
should presumably be that type deduction fails and overload
resolution selects the non-template function. However, the list
of reasons in 14.8.2
temp.deduct for which type
deduction can fail does not include function and array types as a
function return type. Those cases should be added to the
list.

Proposed resolution (October, 2005):

Change the last sub-bullet of 14.8.2
temp.deduct
paragraph 2 as indicated:

Attempting to create a function type in which a parameter
has a type of void, or in which the return type is a
function type or array type.

352.
Nondeduced contexts

The current definition of the C++ language speaks about nondeduced
contexts only in terms of deducing template arguments from a type
14.8.2.5
temp.deduct.type paragraph 4. Those cases, however, don't
seem to be the only ones when template argument deduction is not
possible. The example below illustrates that:

Here argument-dependent name lookup finds the function template
'A::foo' as a candidate function. None of the function template's
function parameter types constitutes a nondeduced context as per
14.8.2.5
temp.deduct.type paragraph 4. And yet, quite clearly,
argument deduction is not possible in this context. Furthermore it is
not clear what a conforming implementation shall do when the
definition of the non-template function '::tfoo' is uncommented.

Suggested resolution:

Add the following as a new paragraph immediately before paragraph 3 of
14.8.2.1
temp.deduct.call:

After the above transformations, in the event of P being a function
type, a pointer to function type, or a pointer to member function type
and the corresponding A designating a set of overloaded (member)
functions with at least one of the (member) functions introduced by
the use of a (member) function template name (13.4
over.over)
or by the use of a
conversion function template (14.8.2.3
temp.deduct.conv),
the whole function call
expression is considered to be a nondeduced context. [Example:

The nested-name-specifier of a type that was specified using a
qualified-id.

A type that is a template-id in which one or more of the
template-arguments is an expression that references a
template-parameter.

with:

The nondeduced contexts are:

The nested-name-specifier of a type that was specified using a
qualified-id.

A non-type template argument or an array bound that is an expression
that references a template-parameter.

A template parameter used in the parameter type of a function parameter
that has a default argument that is being used in the call
for which argument deduction is being done.

A function parameter for which argument deduction cannot be done
because the associated function argument is a function, or a set of
overloaded functions (13.4
over.over), and one or
more of the following apply:

more than one function matches the function parameter type
(resulting in an ambiguous deduction), or

no function matches the function parameter type, or

the set of functions supplied as an argument contains one or more
function templates.

When P is a function type, pointer to function type, or pointer to
member function type:

If the argument is an overload set containing one or more function templates,
the parameter is treated as a nondeduced context.

If the argument is an overload set (not containing function templates),
trial argument deduction is attempted using each of the members of the set.
If deduction succeeds for only one of the overload set members, that member
is used as the argument value for deduction. If deduction succeeds for more
than one member of the overload set the parameter is treated as a nondeduced
context.

If, in the declaration of a function template with a non-type
template-parameter, the non-type template-parameter is
used in an expression in the function parameter-list, the
corresponding template-argument must always be explicitly
specified or deduced elsewhere because type deduction would otherwise
always fail for such a template-argument.

With:

If, in the declaration of a function template with a non-type
template parameter, the non-type template parameter is
used in an expression in the function parameter list, the
expression is a nondeduced context.

A template-argument can be deduced from a pointer to function
or pointer to member function argument if the set of overloaded
functions does not contain function templates and at most
one of a set of overloaded functions provides a unique match.

With:

A template-argument can be deduced from a function,
pointer to function, or pointer to member function type.

In terms of the process described in 14.8.2.1
temp.deduct.call,
P is const charT* const * and A is
char*[1]. According to the first bullet in paragraph 2,
the type used in deduction is not A but “the pointer
type produced by the array-to-pointer standard conversion.”

According to paragraph 4,

In general, the deduction process attempts to find template argument
values that will make the deduced A identical to A
(after the type A is transformed as described
above). However, there are three cases that allow a difference:

In this example, the deduced A is not
identical to the transformed A, because the deduced
A has additional cv-qualification, so the three exceptions
must be examined to see if they apply. The only one that might
apply is the second bullet of paragraph 4:

A can be another pointer or pointer to member type
that can be converted to the deduced A via a qualification
conversion (4.4
conv.qual).

However, A is not a pointer type but an array type;
this provision does not apply and deduction fails.

It has been argued that the phrase “after the type A
is transformed as described above” should be understood to
apply to the A in the three bullets of paragraph 4. If
that is the intent, the wording should be changed to make that
explicit.

In general, the deduction process attempts to find template
argument values that will make the deduced A identical
to A (after the type A is transformed as described
above). However, there are three cases that allow a difference:

If the original P is a reference type, the
deduced A (i.e., the type referred to by the reference) can
be more cv-qualified than the transformedA.

The transformedA can be another pointer or
pointer to member type that can be converted to the deduced A
via a qualification conversion (4.4
conv.qual).

If P is a class, and P has the
form template-id, then the transformedA can be
a derived class of the deduced A. Likewise, if P is
a pointer to a class of the form template-id, the
transformedA can be a pointer to a derived class pointed
to by the deduced A.

If A is a cv-qualified type, the top level cv-qualifiers
of A's type are ignored for type deduction. If
A is a reference type, the type referred to by
A is used for type deduction.

to

If A is a cv-qualified type, the top level cv-qualifiers
of A's type are ignored for type deduction. If A is a
reference type, the type referred to by A is used for
type deduction. If P is a reference type, the type
referred to by P is used for type deduction.

We ran into an issue concerning qualification conversions when doing
template argument deduction for conversion functions.

The question is: What is the type of T in the conversion functions
called by this example? Is T "int" or "const int"?

If T is "int", the conversion function in class A works and the one in
class B fails (because the return expression cannot be converted to
the return type of the function). If T is "const int", A fails and B
works.

Because the qualification conversion is performed on the result of the
conversion function, I see no benefit in deducing T as const int.

In addition, I think the code in class A is more likely to occur than
the code in class B. If the author of the class was planning on returning
a pointer to a const entity, I would expect the function to have been
written with a const in the return type.

We have just implemented this feature, and pending clarification by
the committee, we deduce T as int. It appears that g++ and the Sun
compiler deduce T as const int.

One way or the other, I think the standard should be clarified to
specify how cases like this should be handled.

Notes from October 2002 meeting:

There was consensus on having the deduced type be "int" in the
above.

Proposed resolution (April 2003):

Add to the end of 14.8.2.3
temp.deduct.conv (as a new paragraph
following paragraph 3):

When the deduction process requires a qualification conversion for a
pointer or pointer to member type as described above, the following
process is used to determine the deduced template argument values:

If A is a type cv1,0 pointer to ... cv1,n-1
pointer to cv1,nT1

and P is a type cv2,0
pointer to ... cv2,n-1
pointer to cv2,nT2

The cv-unqualified T1 and T2 are used as the types of
A and P respectively for type deduction.

70.
Is an array bound a nondeduced context?

Paragraph 4 lists contexts in which template formals are not deduced.
Were template formals in an expression in the array bound of an array type
specification intentionally left out of this list? Or was the intent
that such formals always be explicitly specified? Otherwise I believe the
following should be valid:

Expressions involving nontype template parameters are nondeduced
contexts, even though they are omitted from the list in
14.8.2.5
temp.deduct.type
paragraph 4. See
14.8.2.5
temp.deduct.type
paragraphs
12-14:

A template type argument cannot be deduced from the type of a non-type
template-argument.

...

If, in the declaration of a function template with
a non-type template-parameter, the non-type template-parameter
is used in an expression in the function parameter-list, the corresponding
template-argument must always be explicitly specified or deduced
elsewhere because type deduction would otherwise always
fail for such a template-argument.

300.
References to functions in template argument deduction

Paragraph 9 of 14.8.2.5
temp.deduct.type enumerates the
forms that the types P and A need to have in order for template
argument deduction to succeed.

For P denoting a pointer to function the paragraph lists the following
forms as allowing for template argument deduction:

type(*)(T)
T(*)()
T(*)(T)

On the other hand, no provision has been made to accommodate similar
cases for references to functions, which in light of the wording of
14.8.2.5
temp.deduct.type paragraph 11 means that the program below
is ill-formed (some of the C++ compilers do not reject it however):

The nested-name-specifier of a type that was specified
using a qualified-id.

A non-type template argument or an array bound that is an
expression that references a template parameter.

A template parameter used in the parameter type of a function
parameter that has a default argument that is being used in the call
for which argument deduction is being done.

A function parameter for which argument deduction cannot be
done because the associated function argument is a function, or a set
of overloaded functions (13.4
over.over), and one or more
of the following apply:

more than one function matches the function parameter type
(resulting in an ambiguous deduction), or

no function matches the function parameter type, or

the set of functions supplied as an argument contains one or
more function templates.

An array bound that is an expression that references
a template-parameter.

There are two problems with this list:

The last bullet is redundant with the second bullet. This
appears to have been the result of applying the resolutions of issues
70 and 352
independently instead of in coordination.

The second bullet
appears to be contradicted by the statement in paragraph 8 saying that
an argument can be deduced if P and A have the
forms type[i] and
template-name<i>.

The intent of the wording in bullet 2 appears to have been that
deduction cannot be done if the template parameter is a sub-expression
of the template argument or array bound expression and that it can be
done if it is the complete expression, but the current wording does
not say that very clearly. (Similar wording also appears in
14.6.2.1
temp.dep.type paragraph 3 and 14.8.2.5
temp.deduct.type paragraph 14.)

The nested-name-specifier of a type that was specified
using a qualified-id.

A non-type template argument or an array bound that is an
expression thatin either of which a subexpression
references a template parameter.

A template parameter used in the parameter type of a function
parameter that has a default argument that is being used in the call
for which argument deduction is being done.

A function parameter for which argument deduction cannot be done
because the associated function argument is a function, or a set of
overloaded functions (13.4
over.over), and one or more of
the following apply:

more than one function matches the function parameter type
(resulting in an ambiguous deduction), or

no function matches the function parameter type, or

the set of functions supplied as an argument contains one or more function templates.

An array bound that is an expression that references a
template-parameter.

If, in the declaration of a function template with a non-type template
parameter, the non-type template parameter is used in an
expressiona subexpression in the function parameter list,
the expression is a non-deduced context as specified above...

A template argument that is equivalent to a template parameter (i.e.,
has the same constant value or the same type as the template
parameter) can be used in place of that template parameter in a
reference to the current instantiation. In the case of a non-type
template argument, the argument must have been given the value of the
template parameter and not an expression involvingthat
contains the template parameter as a subexpression...

415.
Template deduction does not cause instantiation

Mike Miller:
In fact, now that I've looked more closely, that appears not to be the
case. (At least, it's not the error I get when I compile his example.)
Here's a minimal extract (without the inflammatory using-directive :-)
that illustrates what I think is going on:

What happens is that iterator_traits<int>
is instantiated as part of
type deduction for the function template distance,
and the instantiation
fails. (Note that it can't be instantiation of distance<int>,
as I had originally posited, because in this case only a declaration, not a
definition, of that template is in scope.)

John Spicer: Yes, I believe that is what is going on.

Mike Miller:
I seem to recall that there was some discussion of questions related
to this during the core meetings in Oxford. I think Steve Adamczyk
said something to the effect that it's infeasible to suppress all
instantiation errors during template type deduction and simply call
any such errors a deduction failure. (I could be misremembering, and
I could be misapplying that comment to this situation.)

John Spicer:
Regardless of other conditions in which this may apply, I don't think
it would be reasonable for compilers to have to do "speculative
instantiations" during template argument deduction. One class
instantiation could kick off a series of other instantiations, etc.

Mike Miller:
I don't see anything in the Standard that tells me whether it's
legitimate or not to report an error in this case. I hope John or
another template expert can enlighten me on that.

John Spicer:
My opinion is that, because this case is not among those enumerated
that cause deduction failure (rather than being ill-formed) that
reporting an error is the right thing to do.

Mike Miller:
I am still interested, though, in the question of why
14.8.3
temp.over says that
viable function template specializations are instantiated, even if they
are not selected by overload resolution.

John Spicer:
I believe the wording in
14.8.3
temp.over is incorrect. I researched this and
found that a change was made during the clause 14 restructuring that
was incorporated in March of 1996. The prior wording was "the deduced
template arguments are used to generate a single template function".
This was changed to "deduced template arguments are used to
instantiate a single function template specialization". I believe
this resulted from what was basically a global replace of "generate"
with "instantiate" and of "template function" with "function template
specialization". In this case, the substitution changed the meaning.
This paragraph needs reworking.

...For each function template, if the argument deduction and checking
succeeds, the template-arguments (deduced and/or explicit) are used to
instantiatesynthesize the declaration of a single
function template specialization which is added to the candidate
functions set to be used in overload resolution. If, for a given
function template, argument deduction fails, no such function is added
to the set of candidate functions for that template. The complete set
of candidate functions includes all the function templates
instantiated in this waysynthesized declarations and all
of the non-template overloaded functions of the same name. The
function template specializationssynthesized
declarations are treated like any other functions in the remainder
of overload resolution, except as explicitly noted in 13.3.3
over.match.best.

208.
Rethrowing exceptions in nested handlers

Paragraph 7 of 15.1
except.throw discusses which exception
is thrown by a throw-expression with no operand.

May an expression which has been "finished" (paragraph 7) by an
inner catch block be rethrown by an outer catch block?

catch(...) // Catch the original exception
{
try{ throw; } // rethrow it at an inner level
// (in reality this is probably
// inside a function)
catch (...)
{
} // Here, an exception (the original object)
// is "finished" according to 15.1p7 wording
// 15.1p7 says that only an unfinished exception
// may be rethrown.
throw; // Can we throw it again anyway? It is
// certainly still alive (15.1p4).
}

I believe this is ok, since the paragraph says that the exception
is finished when the "corresponding" catch clause exits. However
since we have two clauses, and only one exception, it would seem that
the one exception gets "finished" twice.

An exception is considered caught when a handler for that exception
becomes active (15.3
except.handle). [Note: an
exception can have active handlers and still be considered uncaught if
it is rethrown.]

An exception is considered handled upon entry to a handler.
[Note: the stack will have been unwound at that point.]

to

A handler is considered active when initialization is complete for
the formal parameter (if any) of the catch clause. [Note: the
stack will have been unwound at that point.] Also, an implicit
handler is considered active when std::terminate() or
std::unexpected() is entered due to a throw. A handler is no
longer considered active when the catch clause exits or when
std::unexpected() exits after being entered due to a throw.

The exception with the most recently activated handler that is
still active is called the currently handled exception.

The type of the throw-expression shall not be an
incomplete type, or a pointer or reference
to an incomplete type other than (possibly cv-qualified)
void, other than void*,
const void*, volatile void*, or
const volatile void*.

479.
Copy elision in exception handling

I have noticed a couple of confusing and overlapping passages
dealing with copy elision. The first is 15.1
except.throw paragraph 5:

If the use of the temporary object can be eliminated without
changing the meaning of the program except for the execution of
constructors and destructors associated with the use of the
temporary object (12.2
class.temporary), then the exception
in the handler can be initialized directly with the argument of
the throw expression.

If the use of a temporary object can be eliminated without
changing the meaning of the program except for execution of
constructors and destructors associated with the use of the
temporary object, then the optional name can be bound directly to
the temporary object specified in a throw-expression
causing the handler to be executed.

I think these two passages are intended to describe the
same optimization. However, as is often the case where something
is described twice, there are significant differences. One is
just different terminology — is “the exception in the
handler” the same as “the object declared in the
exception-declaration or, if the
exception-declaration does not specify a name, a temporary
object of that type” (15.3
except.handle paragraph
16)?

More significant, there is a difference in which kinds of
throw-expressions are eligible for the optimization. In
15.1
except.throw paragraph 5, it appears that any
object is a candidate, while in 15.3
except.handle
paragraph 17 the thrown object must be
a temporary (“the temporary object specified in a
throw-expression”). For example, it's not clear
looking at these two passages whether the copy of a local
automatic can be elided. I.e., by analogy with the return value
optimization described in 12.8
class.copy paragraph
15:

Which brings up another point: 12.8
class.copy
paragraph 15 purports to be an exhaustive list in which copy
elision is permitted even if the constructor and/or destructor
have side effects; however, these two passages describe another
case that is not mentioned in 12.8
class.copy
paragraph 15.

A final point of confusion: in the unoptimized abstract
machine, there are actually two copies in throwing and
handling an exception: the copy from the object being thrown to
the exception object, and the copy from the exception object to
the object or temporary in the exception-declaration.
15.1
except.throw paragraph 5 speaks only of eliminating
the exception object, copying the thrown object directly into the
exception-declaration object, while 15.3
except.handle paragraph 17 refers to directly binding the
exception-declaration object to the thrown object (if it's
a temporary). Shouldn't these be separated, with a throw of an
automatic object or temporary being like the return value
optimization and the initialization of the object/temporary in
the exception-declaration being a separate optimizable
step (which could, presumably, be combined to effectively alias
the exception-declaration onto the thrown object)?

(See paper J16/04-0165 = WG21 N1725.)

Proposed resolution (April, 2005):

Add two items to the bulleted list in 12.8
class.copy
paragraph 15 as follows:

This elision of copy operations is permitted in the following
circumstances (which may be combined to eliminate multiple copies):

in a return statement in a function with a class return type,
when the expression is the name of a non-volatile automatic object
with the same cv-unqualified type as the function return type, the
copy operation can be omitted by constructing the automatic object
directly into the function’s return value

in a throw-expression, when the operand is the name
of a non-volatile automatic object, the copy operation from the
operand to the exception object (15.1
except.throw) can be
omitted by constructing the automatic object directly into the
exception object

when a temporary class object that has not been bound to a
reference (12.2
class.temporary) would be copied to a class
object with the same cv-unqualified type, the copy operation can be
omitted by constructing the temporary object directly into the target
of the omitted copy

when the exception-declaration of an exception
handler (clause 15
except) declares an object of the
same type (except for cv-qualification) as the exception object
(15.1
except.throw), the copy operation can be omitted by
treating the exception-declaration as an alias for the
exception object if the meaning of the program will be unchanged
except for the execution of constructors and destructors for the
object declared by the exception-declaration

If the use of the temporary object can be eliminated without
changing the meaning of the program except for the execution of
constructors and destructors associated with the use of the temporary
object (12.2
class.temporary), then the exception in the handler
can be initialized directly with the argument of the throw
expression. When the thrown object is a class object, and
the copy constructor used to initialize the temporary copy is notand the destructor shall be accessible, the program is
ill-formed (even when the temporary object could otherwise be
eliminated)even if the copy operation is elided (12.8
class.copy). Similarly, if the destructor for that object is
not accessible, the program is ill-formed (even when the temporary
object could otherwise be eliminated).

If the use of a temporary object can be eliminated without changing
the meaning of the program except for execution of constructors and
destructors associated with the use of the temporary object, then the
optional name can be bound directly to the temporary object specified
in a throw-expression causing the handler to be executed. The
copy constructor and destructor associated with the object shall be
accessible even when the temporary object is eliminatedif the copy operation is elided (12.8
class.copy).

An exception-specification shall appear only on
a function declarator in a function, pointer, reference or pointer to member
declaration or definition.

to:

An exception-specification shall appear only on a function
declarator for a function type, pointer to function type, reference to
function type, or pointer to member function type that is the
top-level type of a declaration or definition, or on such a type
appearing as a parameter or return type in a function declarator.

I suggest that the following definition is added to clause
16
cpp, paragraph 1, after the one for
replacement-list:

identifier-list:

identifieridentifier-list , identifier

This should be repeated again in clause A.14
gram.cpp,
again after the one for replacement-list. It might also be
desirable to include a third repetition in clause 16.3
cpp.replace,
paragraph 9.

Proposed Resolution (Clark Nelson, Dec 2003):

In clause 16
cpp, paragraph 1, immediately
before the definition of replacement-list, add:

identifier-list:

identifieridentifier-list , identifier

If the correct TROFF macros are used, the definition will appear
automatically in appendix A. It doesn't need to be repeated in 16.3p9.

With respect to the question of having the preprocessor description be
synchronized with C99, this would fall into the category of a justified
difference. (Other justified differences include those for Boolean
expressions, alternative tokens, and terminology differences.)

370.
Can #include <...> form be used other than for standard C++ headers?

The motivation for this issue is a desire to write portable programs which
will work with any conforming implementation.

The C++ Standard (16.2
cpp.include) provides two forms of
#include directives, with the <...> form being described
(16.2
cpp.include paragraph 2) as "for a header", and
the "..." form (16.2
cpp.include paragraph 3) as
for "the source file" identified between the delimiters. When the
standard uses the term "header", it often appears to be limiting
the term to apply to the Standard Library headers only. Users of the
standard almost always use the term "header" more broadly, to
cover all #included source files, but
particularly those containing interface declarations.

Headers, including source files, can be categorized according to
their origin and usage:

C++ Standard Library headers (which aren't necessarily files).

Other standard libraries such as the POSIX headers.

Operating system API's such as windows.h.

Third party libraries, such as Boost, ACE, or commercial offerings.

Organization-wide "standard" header files, such as a company's
config.hpp.

A project's "public" header files, often shared by all
developers working on the same project.

A project or user's "private", "local", or
"detail" headers, in the same directory or sub-directory
as the compilation unit.

Existing practice varies widely, but it is fairly easy to find users
advocating:

For (1), Standard Library headers, use <...>. All others
use "...".

For (7), use "...". All others use <...>.

Draw a line somewhere below (1) and above (7), and use
<...> above the line and "..." below.
The exact location of the line may be based on a perception of
how often the header involved changes; some "make" utilities
use this to optimize builds.

Do any of the practices A, B, or C result in programs which can be rejected
by a conforming implementation?

The first defect is that readers of the standard have not been
able to reach consensus on the answers to the above question.

A second possible defect is that if A, B, or C can be rejected by a
conforming implementation, then the standard should be changed because would
mean there is a wide variance between the standard and existing practice.

Matt Austern:
I really only see two positions:

Implementations have some unspecified mechanism (copying
files to a magic directory, adding a magic flag,...) such that the
line #include <foo> results in textual inclusion of
the file foo.

Implementations are not required to have any mechanism for
getting #include <foo> to perform textual inclusion of an
arbitrary file foo. That form is reserved for standard
library headers only.

I agree that the standard should clarify which of those two is the
case (I imagine it'll hinge on finding one crucual sentence that
either says "implementation defined" or "unspecified"), but from
the standpoint of portability I don't see much difference between
the two. I claim that, with either of those two interpretations, using
#include <foo> is already nonportable.

(Of course, I claim that almost anything having to do with headers,
including the #include "foo" form, is also nonportable. In practice
there's wide variation in how compilers handle paths, especially
relative paths.)

Beman Dawes:
The whole issue can be resolved by replacing "header" with "header or
source file" in 16.2
cpp.include paragraph 2.
That will bring the standard into
alignment with existing practice by both users and implementations.
The "header and/or source file" wording is used at least three other
places in the standard where otherwise there might be some confusion.

John Skaller:
In light of Andrew Koenig's comments, this doesn't appear to be the case,
since the mapping of include names to file names is implementation defined,
and therefore source file inclusion cannot be made portable
within the ISO C/C++ standards (since that provision obviously cannot be
removed).

A possible idea is to create a binding standard, outside the
C/C++ ISO Standards, which specifies not only the path lookup
mechanism but also the translation from include names to file
names. Clearly that is OS dependent, encoding dependent, etc,
but there is no reason not to have a binding standard for Unix,
Windows, etc, and specify these bindings in such a way that
copying directories from one OS to the other can result in
programs working on both OS's.

Andy Koenig:
An easier solution might be to specify a (presumably unbounded, or
bounded only by implementation capacity) collection of header-file
names that every implementation must make it possible for programs
to access somehow, without specifying exactly how.

Notes from October 2002 meeting:

This was discussed at some length. While there was widespread
agreement that such inclusion is inherently implementation-dependent,
we agreed to try to add wording that would make it clear that
implementations are permitted (but not required) to allow inclusion
of files using the <...> form of #include.

[Example: The most common uses of #include preprocessing
directives are as in the following:

#include <stdio.h>
#include "myprog.h"

—end example]

to:

[Note: Although an implementation may provide a mechanism for
making arbitrary source files available to the < >
search, in general programmers should use the < > form
for headers provided with the implementation, and the " "
form for sources outside the control of the implementation. For
instance:

Some doubt was expressed as to whether the benefit of this
non-normative clarification outweighs the overall goal of synchronizing
clause 16 with the corresponding text in the C99 Standard. As a
result, this issue is being left in “review” status to
allow further discussion.

If a typedef TD names a type "reference to
cv1S," an attempt to create the type
"reference to cv2TD" creates the type
"reference to cv12"S," where cv12 is the
union of the cv-qualifiers cv1 and cv2.
Redundant qualifiers are ignored. [Example:

If a template-argument for a template-parameterT names a type "reference to cv1S,"
an attempt to create the type "reference to cv2T"
creates the type "reference to cv12S," where
cv12 is the union of the cv-qualifiers cv1 and
cv2. Redundant cv-qualifiers are ignored.
[Example:

41.
Clarification of lookup of names after declarator-id

Footnotes 26 and 29 both use the phrase "following the function declarator"
incorrectly: the function declarator includes the parameter list, but the
footnotes make clear that they intend what's said to apply to names inside
the parameter list. Presumably the phrase should be "following the function
declarator-id."

A name used in the definition of a function [footnote: This
refers to unqualified names following the function declarator; such a name
may be used as a type or as a default argument name in the
parameter-declaration-clause,
or may be used in the function body. end footnote] that is ...

to:

A name used in the definition of a function following the function's
declarator-id [footnote:
This refers to unqualified names that occur, for instance, in a type or
default argument expression in the parameter-declaration-clause
or used in the function body. end footnote] that is ...

A name used in the definition of a function that is a member
function (9.3
class.mfct
)
[footnote: That is, an unqualified name following the function
declarator; such a name may be used as a type or as a default argument
name in the parameter-declaration-clause, or may be used in the function
body, or, if the function is a constructor, may be used in the expression
of a mem-initializer. end footnote] of class X shall be ...

to:

A name used in the definition of a member function
(9.3
class.mfct
) of
class X following the function's declarator-id
[footnote:
That is, an unqualified name that occurs, for instance, in a type or default
argument expression in the parameter-declaration-clause, in the
function body, or in an expression of a mem-initializer in a constructor
definition.
end footnote] shall be ...

33.
Argument dependent lookup and overloaded functions

If an argument used for lookup is the address of a group of overloaded
functions, are there any associated namespaces or classes? What if it's
the address of a function template?

My inclination is to say no to both.

From Mike Miller:

We discussed this on the reflector a few weeks ago. I'll leave the template
case for the Core III experts, but I'd find it surprising if the overload
case weren't handled as the obvious generalization of the single-function
case. For a single function, the associated namespaces are those of the
types used in the parameters and return type; I would expect that using
an overloaded function name would simply be the union of the namespaces
from the members of the overload set. That would be the simplest and most
intuitive, IMHO — is there an argument for doing it differently?

Proposed Resolution (04/99):
In 3.4.2
basic.lookup.argdep
paragraph 2,
add following the last bullet in the list of associated classes and namespaces
for various argument types (not a bullet itself because overload sets and
templates do not have a type):

In addition, if the argument is the name or address of a set
of overloaded functions and/or function templates, its associated classes
and namespaces are the union of those associated with each of the members
of the set: the namespace in which the function or function template is
defined and the classes and namespaces associated with its (non-dependent)
parameter types and return type.

90.
Should the enclosing class be an "associated class" too?

If T is a class type, its associated classes are the class itself and its
direct and indirect base classes. Its associated namespaces are the namespaces
in which its associated classes are defined.

If T is a union or enumeration type, its associated namespace is the namespace
in which it is defined. If it is a class member, its associated class is
the member's class; else it has no associated class.

Note that for a union, the enclosing class is an "associated class", but
for a class type the enclosing class is not an "associated class". This
results in some surprising behavior, as shown in the example below.

Certainly the enclosing class should also be an associated class for nested
class types, shouldn't it?

Proposed Resolution (10/99):
Change the two referenced bullets to read:

If T is a class type (including unions),
its associated classes are:
the class itself; the
class of which it is a member, if any; and its direct and indirect base
classes. Its associated namespaces are the namespaces in which its
associated classes are defined.

If T is an enumeration type,
its associated namespace is the namespace in
which it is defined. If it is class member, its associated class is the
member's class; else it has no associated class.

164.
Overlap between Koenig and normal lookup

...other namespaces not considered during the usual unqualified lookup
(3.4.1
basic.lookup.unqual
) may be
searched.

Does this mean that Koenig lookup does not search namespaces
that were already searched during the usual unqualified lookup? The
answer is academic except for the two-stage lookup during template
instantiation. If a given namespace is searched in the context of the
template definition, are declarations in that namespace in the
instantiation context ignored during the Koenig lookup? For instance,

43.
Copying base classes (PODs) using memcpy

For any complete POD object type T, whether or not the object
holds a valid value of type T, the underlying bytes
(1.7
intro.memory
) making
up the object can be copied into an array of char
or unsigned char*. If the content of
the array of char or unsigned char is copied back into the object, the
object shall subsequently hold its original value. [Example elided]

*[Footnote: By using, for example, the library functions
(17.4.1.2
lib.headers
)
memcpy or memmove. end footnote]

For any POD type T, if two pointers to T point
to distinct T objects obj1
and obj2, if the value of obj1 is copied into
obj2, using the memcpy library
function, obj2 shall subsequently hold the same value as
obj1.

Paragraph 3 doesn't repeat the restriction of paragraph 2. Should it be
assumed? Otherwise only complete POD types are copyable to an array of
char and back, but scribbling over subobjects is OK.
(Or perhaps a "distinct T object" is a complete object...)

not involving conversions to pointers to private or
protected or ambiguous classes

The phrase "unambiguous public base" is unfortunate as it
could mean either "an
unambiguous base not considering accessibility, which is public"
or "an
unambiguous base considering only the publicly accessible bases."
I believe the
former interpretation correct, as accessibility is applied after visibility
(11
class.access
paragraph 4)
and ambiguity is described in terms of visibility
(10.2
class.member.lookup
paragraph 2).

Suggested Resolution:
Use the phrases "public and unambiguous,"
"accessible and unambiguous,"
"non-public or ambiguous," or "inaccessible or ambiguous" as appropriate.

10.3
class.virtual
paragraph 5: change
"the class in the return type... is an unambiguous direct or
indirect base class... and is accessible in D" to "the
class in the return type... is an unambiguous and accessible direct
or indirect base class..."

123.
Bad cross-reference

Section: 5.1
expr.primStatus: TC1
Submitter: Mike Miller
Date: 3 June 1999
The cross-reference is incorrect in the first
sentence after the grammar in
5.1
expr.prim
paragraph 7:

A nested-name-specifier that names a class, optionally
followed by the keyword template
(14.8.1
temp.arg.explicit
), ...

147.
Naming the constructor

5.1
expr.prim
paragraph 7 says that
class-name::class-name names the constructor when both
class-name refer to the same class. (Note the different
perspective, at least, in
12.1
class.ctor
paragraph 1, in which
constructors have no names and are recognized by syntactic context
rather than by name.)

This formulation does not address the case of classes in which a
function template is declared as a constructor, for example:

Here there is an ambiguity as to whether the second template argument
list is for the injected class name or for the constructor.

Suggested resolution: restate the rule as a component of name
lookup. Specifically, if when doing a qualified lookup in a given
class you look up a name that is the same as the name of the class,
the entity found is the constructor and not the injected class name.
In all other cases, the name found is the injected class name. For
example:

Here the declarator for the definition of the static data member
has redundant parentheses, and it's only after seeing the declarator
that the parser can know that the second A<int> is the
injected class name rather than the constructor.

If the nested-name-specifier nominates a class C, and the
name specified after the nested-name-specifier, when looked up in
C, is the injected-class-name of C (clause
9
class), the name is instead considered
to name the constructor of class C. Such a constructor name
shall only be used in the declarator-id of a constructor
definition that appears outside of the class definition.
[Example:

52.
Non-static members, member selection and access checking

5.2.5
expr.ref
paragraph 4 should make
it clear that when a nonstatic member is referenced in a member
selection operation, the type of the left operand is implicitly cast
to the naming class of the member. This allows for the detection of
access and ambiguity errors on that implicit cast.

If the member m is accessible when named in the naming
class according to the rules below, the access to m is
nonetheless ill-formed if the type of p cannot be
implicitly converted to type T (for example, if T is an
inaccessible base class of p's class).

If a class member access operator, including an
implicit "this->," is used to access a nonstatic
data member or nonstatic member function, the
reference is ill-formed if the left operand
(considered as a pointer in the "." operator case)
cannot be implicitly converted to a pointer to the
naming class of the right operand. [Note: this
requirement is in addition to the requirement that the
member be accessible as named.]

In 11.2
class.access.base paragraph 4, fix a typographical
error by adding the missing right parenthesis following the text

If the function is a nonstatic member function, the
"this" parameter of the function (9.3.2
class.this) shall
be initialized with a pointer to the object of the
call, converted as if by an explicit type conversion
(5.4
expr.cast). [Note: there is no access checking on
this conversion; the access checking is done as part of
the (possibly implicit) class member access operator.
See 11.2
class.access.base.]

53.
Lvalue-to-rvalue conversion before certain static_casts

Section 5.2.9
expr.static.cast
paragraph 6
should make it clear that when any of the
"inverse of any standard conversion sequence" static_casts are done, the
operand undergoes the lvalue-to-rvalue conversions first.

can be performed explicitly using static_cast subject to the
restriction that the explicit conversion does not cast away constness
(5.2.11
expr.const.cast), ...

to

can be performed explicitly using static_cast. The
lvalue-to-rvalue (4.1
conv.lval), array-to-pointer
(4.2
conv.array), and function-to-pointer (4.3
conv.func) conversions are applied to the operand. Such a
static_cast is subject to the restriction that it does
not cast away constness (5.2.11
expr.const.cast), ...

137.
static_cast of cvvoid*

An rvalue of type "pointer to cvvoid" can be explicitly
converted to a pointer to object type.

No requirements are stated regarding the cv-qualification of the
pointer to object type. Contrast this with the formula used in
paragraphs 5, 8, and 9, where the treatment of cv-qualification
is explicit, requiring that the target type be at least as
cv-qualified as the source.
There is an apparently general requirement on all forms of
static_cast in
5.2.9
expr.static.cast
paragraph 1 that it
"shall not cast away constness."
Assuming that this restriction applies to paragraph 10, since there is
no explicit exception to the general rule, that still leaves
open the question of whether one can "cast away volatility"
in a conversion from volatile void* to a pointer to object
type.
Should 5.2.9
expr.static.cast
paragraph 10
be rewritten to handle cv-qualification in
the same way as paragraphs 5, 8, and 9?

An rvalue of type "pointer to cv1void" can be
converted to an rvalue of type "pointer to cv2T", where
T is an object type and cv2 is the same cv-qualification
as, or greater cv-qualification than, cv1.

127.
Ambiguity in description of matching deallocation function

If a placement allocation function has default arguments for all its
parameters except the first, it can be called using non-placement
syntax. In such a case, it is not clear whether
the deallocation function to be called if the constructor terminates
by throwing an expression is determined on the basis of
the syntax of the new-expression (i.e., a non-placement
deallocation function)
or the declaration of the selected (placement) allocation function.
5.3.4
expr.new
paragraph 19 indicates
that the deallocation function must match the declaration of the
allocation function.
However, 15.2
except.ctor
says that the
distinction is based on whether the new-expression contains
a new-placement or not.

If the object or array was allocated in a new-expression and
the new-expression does not contain a new-placement, the
deallocation function (3.7.3.2
basic.stc.dynamic.deallocation, 12.5
class.free) is called to free the storage occupied by the object;
the deallocation function is chosen as specified in 5.3.4
expr.new. If the object or array was allocated in a
new-expression and the new-expression contains a
new-placement, the storage occupied by the object is
deallocated only if an appropriate placement operator delete is found,
as specified in 5.3.4
expr.new.

179.
Function pointers and subtraction

If two pointers point to the same object or function... and the
two pointers are subtracted...

However, 5.7
expr.add
paragraph 2
requires that two pointers that are subtracted be pointers to an
object type; function pointers are not allowed.

Being able to subtract two pointers to functions doesn't seem
terribly useful, especially considering that subtracting two pointers
to different functions appears to produce undefined behavior rather
than simply a non-zero result, according to paragraph 6:

Unless both pointers point to elements of the same array object, or
one past the last element of the array object, the behavior is
undefined.

Proposed resolution (10/00):

Remove the words or function
from paragraph 8.

73.
Pointer equality

Pointers to objects or functions of the same type (after pointer
conversions) can be compared for equality. Two pointers of the same type
compare equal if and only if they are both null, both point to the same
object or function, or both point one past the end of the same array.

What does this say, when we have

int i[1];
int j[1];

about the expression (i+1 == j) ? It seems to require padding
between i[0] and j[0] so that the comparison will come
out false.

I think this may be a defect, in that the quoted paragraph extends
operator=='s domain too far beyond operator<'s. It should permit
(but not require) an off-the-end pointer to compare equal to another
object, but not to any element of the same array.

Mike Miller: I think this is reading more into the statement
in
5.10
expr.eq
paragraph 1 than is actually
there. What does it mean for a pointer to "point to" an object?
I can't find anything that definitively says that i+1 cannot "point
to" j[0] (although it's obviously not required to do so).
If i+1 is allowed to "point to" j[0], then i+1==j
is allowed to be true, and there's no defect. There are places where
aliasing is forbidden, but the N+1th element of an array doesn't appear
to be one of them.

To put it another way, "points to" is undefined in the Standard. The
only definition I can think of that encompasses the possible ways in which
a pointer can get its value (e.g., the implementation-defined conversion
of an arbitrary integer value to a pointer) is that it means "having the
same value representation as would be produced by applying the (builtin)
& operator to an lvalue expression designating that object".
In other words, if the bits are right, it doesn't matter how you produced
the value, as long as you didn't perform any operations that have undefined
results. The expression i+1 is not undefined, so if the
bits of i+1 are the same as those of &j[0], then
i+1 "points to" j[0] and i+i==j is allowed to
be true.

Tom MacDonald: C9X contains the following words for the "=="
operator:

Two pointers compare equal if both are null pointers, both
are pointers to the same object (including a pointer to an object and a
subobject at its beginning) or function, both are pointers to one
past the last element of the same array object, or one is a pointer to
one past the end of one array object and the other is a pointer to the
start of a different array object that happens to immediately follow the
first array object in the address space.

Matt Austern: I don't think there's anything wrong with saying that
the result of

int x[1];
int y[1];
std::cout << (y == x + 1) << std::endl;

is implementation defined, or even that it's undefined.

Mike Miller: A similar question could be raised about different
objects that (sequentially) share the same storage. Consider the following:

Section
3.8
basic.life
paragraph 5 does
not list this kind of comparison among the pointer operations that cause
undefined behavior, so presumably the comparison is allowed. However,
5.10
expr.eq
paragraph 1 describes pointer comparison in terms of "[pointing] to the
same object," which bp1 and bp2 clearly do not do. How
should we describe the result of this comparison?

Jason Merrill:
When you consider comparing pointers to void, this seems to suggest that no
two objects can have the same address, depending on your interpretation of
"point to the same object." This would cripple the empty base
optimization.

3.9.2
basic.compound
refers to 'pointers
to void or objects or functions'. In that case,
5.10
expr.eq
does
not allow you to compare them; it only allows comparing pointers to
objects and functions.

Proposed Resolution (10/00):

In 3.9.2
basic.compound paragraph 3, add the following
wording immediately preceding, "The value representation of pointer
types is implementation-defined":

A valid value of an object pointer type represents either the address
of a byte in memory (1.7
intro.memory) or a null pointer
(4.10
conv.ptr). If an object of type T is
located at an address A, a pointer of type cvT* whose value is the address A is said to point
to that object, regardless of how the value was obtained.
[Note: for instance, the address one past the end of an array
(5.7
expr.add) would be considered to point to an
unrelated object of the array's element type that might be located at
that address.]

In 5.10
expr.eq paragraph 1, change the sentence
beginning, "Two pointers of the same type..." to read:

Two pointers of the same type compare equal if and only if they are
both null, both point to the same function, or both represent the same
address (3.9.2
basic.compound).

(See also paper J16/00-0011 = WG21 N1234.)

188.
Comma operator and rvalue conversion

What does the sizeof expression return? According to
5.18
expr.comma
paragraph 1,
the comma operator yields
an lvalue if the second argument is an lvalue. Since
4.2
conv.array
paragraph 1 says that the
array-to-pointer conversion yields an rvalue, it seems that
sizeof should see an array type and give the answer 100.
If so, the value of the sizeof expression would be
different from that of the corresponding expression in C, but there
is nothing in Annex
C
diff
to indicate that an incompatible change was intended.

Change: The result of a conditional expression, an
assignment expression, or a comma expression may be an lvalue.Rationale: C++ is an object-oriented language, placing
relatively more emphasis on lvalues. For example, functions may return
lvalues.Effect on original feature: Change to semantics of well-defined
feature. Some C expressions that implicitly rely on lvalue-to-rvalue
conversions will yield different results. For example,

char arr[100];
sizeof(0, arr)

yields 100 in C++ and sizeof(char*) in C.Difficulty of converting: Programs must add explicit casts to
the appropriate rvalue.How widely used: Rare.

94.
Inconsistencies in the descriptions of constant expressions

According to
9.4.2
class.static.data
paragraph 4, a static const integral or const enumeration data member initialized
with an integral constant expression "can appear in integral constant expressions
within its scope" [emphasis mine]. This means that the following
is not permitted:

Is this restriction intentional? If so, what was the rationale for the
restriction?

Bjarne Stroustrup: I think that once you have said S::,
c is in scope so that

int a[S::c];

is ok.

Mike Miller: I'd like to think that's what it meant, but I
don't believe that's what it said. According to
3.3
basic.scope
paragraph 1, the scope of
a name is the region "in which that name may be used as an unqualified
name." You can, indeed, use a qualified name to refer to a name that
is not in scope, but that only goes to reinforce my point that
"S::c" is not in scope at the point where the expression
containing it is used. I think the phrase "within its scope" is at
best misleading and should be removed. (Unless there's a reason I'm
missing for restricting the use of static member constants to their
scope.)

According to 5.19
expr.const
paragraph
1, integral constant expressions can "involve...const variables or static
data members of integral or enumeration types initialized with constant
expressions." However, in 5.19
expr.const
paragraph 3, arithmetic constant expressions cannot include them. This
seems a rather gratuitous distinction and one likely to bite programmers
trained always to use const variables instead of preprocessor definitions.
Again, is there a rationale for the difference?

As far as I can tell from 5.19
expr.const
paragraph 2, "arithmetic constant expressions" (as distinct from "integral
constant expressions") are used only in static initializers to distinguish
between static and dynamic initialization. They include floating point
types and exclude non-type template parameters, as well as the const variables
and static data members.

There is a minor error in 5.19
expr.const
paragraph 2. The first sentence says, "Other expressions are considered
constant expressions only for the purpose of non-local static object initialization."
However, 6.7
stmt.dcl
paragraph 4 appears
to rely on the same definition dealing with the initialization of local
static objects. I think that the words "non-local" should be dropped and
a cross reference to 6.7
stmt.dcl
added.

5.19
expr.const
paragraph 4 says, "An expression
that designates the address of a member or base class of a non-POD class
object (clause 9) is not an address constant expression (12.7
class.cdtor
)."

I'm guessing that should be "non-static member," like the similar
prohibition in 12.7
class.cdtor
regarding
out-of-lifetime access to members of non-POD class objects.

69.
Storage class specifiers on template declarations

Mike Ball: I cannot find anything in the standard that tells me the meaning of
a storage-class-specifier on a function template declaration. In
particular, there is no indication what effect, if any, it has on the storage
class of the instantiations.

There is an explicit prohibition of storage-class-specifiers
on explicit specializations.

For example, if we have

template<class T> static int foo(T) { return sizeof(T); }

does this generate static functions for all instantiations? By
7.1.1
dcl.stc
the storage class applies to the name declared in the declarator, which
is the template foo, not an instantiation of foo, which
is named with a template-id. There is a statement in clause
14 that template names have linkage, which supports the contention that
"static" applies to the template, not to instantiations.

So what does the specifier mean? Lacking a direct statement in
the standard, I see the following posibilities, in my preference order.

storage-class-specifiers have no meaning on template declarations,
their use being subsumed by "export" (for the template name) and
the unnamed namespace (for instantiations)

storage-class-specifiers have no effect on the template name, but
do affect the linkage of the instantiations, though this now applies linkage
to template-ids, which I can find no support for. I suspect
this is what was intended, though I don't remember

Of course, if anybody can find some concrete statement, that would settle
it.

From John Spicer

The standard does say that a namespace scope template has external linkage
unless it is a function template declared "static". It doesn't explicitly
say that the linkage of the template is also the linkage of the instantiations,
but I believe that is the intent. For example, a storage class is
prohibited on an explicit specialization to ensure that a specialization
cannot be given a different storage class than the template on which it
is based.

Mike: This makes sense, but I couldn't find much support
in the document. Sounds like yet another interpretation to add to
the list.

John: Agreed.

The standard does not talk about the linkage of instantiations, because
only "names" are considered to have linkage, and instances are not really
names. So, from an implementation point of view, instances have linkage,
but from a language point of view, only the template from which the instances
are generated has linkage.

Mike: Which is why I think it would be cleaner to eliminate
storage class specifiers entirely and rely on the unnamed namespace.
There is a statement that specializations go into the namespace of the
template. No big deal, it's not something it says, so we live with
what's there.

John: That would mean prohibiting static function templates.
I doubt those are common, but I don't really see much motivation for getting
rid of them at this point.

"export" is an additional attribute that is separate from linkage, but
that can only be applied to templates with external linkage.

Mike: I can't find that restriction in the standard,
though there is one that templates in an unnamed namespace can't be exported.
I'm pretty sure that we intended it, though.

John: I can't find it either. The "inline" case seems to
be addressed, but not static. Surely this is an error as, by definition,
a static template can't be used from elsewhere.

A template name has linkage (3.5
basic.link).
A non-member function template
can have internal linkage; any other template name shall have external
linkage. Entities generated from a template with internal linkage are distinct
from all entities generated in other translation units.

If an elaborated-type-specifier is the sole constituent of a declaration,
the declaration is ill-formed unless it is an explicit specialization
(14.7.3
temp.expl.spec
),
an explicit instantiation
(14.7.2
temp.explicit
) or it has one of the following
forms:

friend declarations refer to functions or classes
that are members of the nearest enclosing namespace ...

I believe it
is intended to mean unqualified friend declarations. Certainly
friend void A::B() need not refer
to a member of the nearest enclosing
namespace. Only when the declarator is unqualified (i.e., it is a declaration
and not a reference) does this rule need to apply. The presence of an
explicit template argument list requires
that a previous declaration be visible and renders this a reference and not
a declaration that is subject to this rule.

When looking for a prior declaration of a
class or a function declared as a friend, scopes outside the
innermost enclosing namespace scope are not considered.

On the other hand, the friend declaration would be a syntax
error if f weren't declared as a template name; it would
seem very strange not to find the declaration that made the
friend declaration syntactically correct.
However, it also seems strange to treat this case differently
from ordinary functions and from templates:

John Spicer:
This section refers to "looking for a prior declaration". This
gets back to an earlier discussion we've had about the difference between
matching two declarations of the same name and doing name lookup. I would
maintain that in f<int>
the f is looked up using a normal lookup. In
practice, this is really how it has to be done because the declaration
could actually be f<int>::x.

When looking for a prior declaration of a class or a function declared
as a friend, scopes outside the innermost enclosing namespace scope are
not considered.

to

When looking for a prior declaration of a class or a function declared
as a friend, and when the name of the friend class or function is
neither a qualified name nor a template-id, scopes outside the
innermost enclosing namespace scope are not considered.

101.
Redeclaration of extern "C" names via using-declarations

According to
7.3.3
namespace.udecl
paragraph 11, the using-declaration is an error:

If a function declaration in namespace scope or block
scope has the same name and the same parameter types
as a function introduced by a using-declaration, the
program is ill-formed.

Based on the context (7.3.3
namespace.udecl
paragraph 10 simply reiterates the requirements
of
3.3
basic.scope
), one might wonder if
the failure to exempt extern "C"
functions was intentional or an oversight. After all, there is
only one function f() involved, because it's extern "C", so
ambiguity is not a reason to prohibit the using-declaration.

This also breaks the relatively strong parallel between extern "C"
functions and typedefs established in our discussion of
Core issue 14
in Santa Cruz. There the question was for using-directives:

We decided for both that there was no ambiguity because each
pair of declarations declares the same entity. (According to
3
basic
paragraph 3, a typedef name is not an entity, but a type is; thus the
declarations of size_t declare the same entity "unsigned int".)

In the context of using-declarations, there is no explicit
extension of the restrictions in 3.3
basic.scope
paragraph 4 except as noted above for function declarations; thus the parallel
scenario for a typedef is not ill-formed:

I think the first sentence of
7.3.3
namespace.udecl
paragraph 11 ought to be rewritten as:

If a function declaration in namespace scope or block
scope has the same name and the same parameter types
as a function introduced by a using-declaration,
and the declarations do not declare the same function, the
program is ill-formed.

Proposed Resolution (10/99):
As suggested.

103.
Is it extended-namespace-definition or extension-namespace-definition ?

If a namespace is extended by an extended-namespace-definition after a
using-directive for that namespace is given, the additional members of
the extended namespace and the members of namespaces nominated by
using-directives in the extended-namespace-definition can be used
after the extended-namespace-definition.

I think the intent is clear, but unfortunately I cannot find any other
mention (or definition) of this term.

40.
Syntax of declarator-id

There are two sub-issues. The first concerns the statement in
8.3
dcl.meaning
paragraph 1,

The id-expression of a declarator-id shall be
a simple identifier except for the declaration of some special functions
(12.3
class.conv
,
12.4
class.dtor
,
13.5
over.oper
) and for the declaration
of template specializations or partial specializations
(14.7
temp.spec
).

The second sub-issue is regarding another statement in the same paragraph:

A declarator-id shall not be qualified except for the
definition of a member function (9.3
class.mfct
)
or static data member (9.4
class.static
)
or nested class (9.7
class.nest
) outside
of its class, the definition or explicit instantiation of a function, variable
or class member of a namespace outside of its namespace, or...

Analysis

The problem in the first sub-issue is that the wrong syntactic non-terminal
is mentioned. The relevant portions of the grammar are:

The exceptions in the citation from
8.3
dcl.meaning
paragraph 1 are all the non-identifier cases of unqualified-id:
12.3
class.conv
is for conversion-function-ids,
12.4
class.dtor
is for destructors,
13.5
over.oper
is
for overloaded operators, and
14.7
temp.spec
is for template-ids. If taken literally, this sentence would exclude
all qualified-ids, which it obviously is not intended to do. Instead,
the apparent intent is something along the lines of

If an unqualified-id is used as the id-expression
of a declarator-id, it shall be a simple identifier except...

However, it does not appear that this restriction has any meaning; all
of the possible cases of unqualified-ids are represented in the
list of exceptions! Rather than recasting the sentence into a correct but
useless form, it would be better to remove it altogether.

The second sub-issue deals with the conditions under which a qualified-id
can be used in a declarator, including "the definition of a...nested class"
and "the definition or explicit instantiation of a...class member of a
namespace." However, the name in a class definition is not part of a declarator;
these constructs do not belong in a list of declarator contexts.

Proposed Resolution for sub-issue 1 (04/99):

The suggested resolution for the first sub-issue overlooked the
fact that the existing wording has the additional effect of prohibiting
the use of the non-identifier syntax for declaring other than the
listed entities. Thus the proposed wording
for the first sub-issue is:

A declarator-id shall not be qualified except for the
definition of a member function (9.3
class.mfct
)
or static data member (9.4
class.static
)
or nested class (9.7
class.nest
) outside
of its class, the definition or explicit instantiation of a function, variable
or class member of a namespace outside of its namespace, or...

to

A declarator-id shall not be qualified except for the
definition of a member function (9.3
class.mfct
)
or static data member (9.4
class.static
)
outside
of its class, the definition or explicit instantiation of a function or
variable member of a namespace outside of its namespace, or...

159.
Namespace qualification in declarators

In the qualified declarator-id for a class or namespace member
definition that appears outside of the member's class or namespace,
the nested-name-specifier shall not name any of the namespaces that
enclose the member's definition.

I was very surprised when this rule was pointed out to me. The change
appears to have been introduced around the time of the first Santa
Cruz meeting, but I don't recall discussion of it and could not find a
motion related to it.

Regardless of where it came from, I also can't understand why it is there.
Certainly it shouldn't matter how you name a given class or namespace.

So, it is okay to use an alias for N::M,
but not to use N::M directly.
Note that it is okay to use N::M
in any other context at this point
in the program (i.e., the rule is a specific restriction on declarator
names, not a general rule on the use of qualified names).

Does anyone recall the intent of this rule or any rationale for its
existence?

Notes from 04/00 meeting:

There was some question as to whether this issue actually
constituted a defect in the Standard. John Spicer suggested that
machine-generated source code would be likely to run afoul of this
prohibition. Francis Glassborow expressed support for a rule that
would allow full qualification, or qualification relative to the
namespace containing the definition, but not qualification relative to
a containing namespace. There was no consensus for moving forward
with a DR at this point, so the issue was left in "review" status.

Proposed resolution (10/00):

Remove the last sentence of
8.3
dcl.meaning paragraph 1 (cited above) and the example
that follows.

135.
Class type in in-class member function definitions

3.2
basic.def.odr
paragraph 4 and
8.3.5
dcl.fct
paragraph 6 indicate that
the return type and parameter types must be complete in a function
definition. However, when
9.2
class.mem
paragraph 2 lists the
contexts in a class member-specification in which the class
is considered complete, the return type and parameter types of a
member function defined in the class definition are not included.
It thus appears that the following example is ill-formed:

Jack Rouse:
I suggest supplementing the text in 8.3.5p6 with something like:

The type of a parameter or the return type for a function definition
shall not be an incomplete class type unless the function definition
is nested in the member-specification for that class (including
definitions in nested classes defined within the class).

Proposed resolution (10/00): Replace the last sentence of
8.3.5
dcl.fct
paragraph 6 with

The type of a parameter or the return type for a function definition
shall not be an incomplete class type (possibly cv-qualified) unless
the function definition is nested within the
member-specification for that class (including definitions in
nested classes defined within the class).

1.
What if two using-declarations refer to the same function but the declarations introduce different default-arguments?

If the best viable function resolves to a function for which
multiple declarations were found, and if at least two of these
declarations — or
the declarations they refer to in the case of
using-declarations — specify
a default argument that made the function viable, the program is ill-formed.
[Example: namespace A { extern "C" void f(int = 5); } namespace B { extern "C" void f(int = 5); }

217.
Default arguments for non-template member functions of class templates

For non-template functions, default arguments can be added in later
declarations of a function in the same scope.

The default arguments in a member function definition that appears
outside of the class definition are added to the set of default
arguments provided by the member function declaration in the class
definition.

This would appear to allow the following example, in which a
default argument is added to a non-template member function of a class
template:

John Spicer:
The wording "non-template
functions" is somewhat unclear with respect to member functions of class
templates, but I know that this was intended to include them because it
originates from issue 3.13 of the template issues list that I maintained for
several years.

Having said that, the rationale for this restriction has since been made
obsolete, so this could (in theory) be changed in the standard if it is
problematic for users.

The default arguments in a member function definition that appears
outside of the class definition are added to the set of default
arguments provided by the member function declaration in the class
definition.

with

Except for member functions of class templates, the default arguments
in a member function definition that appears outside of the class
definition are added to the set of default arguments provided by the
member function declaration in the class definition. Default
arguments for a member function of a class template must be specified
on the initial declaration of the member function within the class
template.

35.
Definition of default-initialization

Once upon a time, we went through a fairly protracted discussion to ensure
that S1().x would be guaranteed to be 0. Note that if we declare

void f()
{
S1 s1;
// ...
}

there is no guarantee of the value of s1.x, and that is intentional.
But S1().x is different, because S1() is an rvalue, and
unless all of its members are defined, the effect of copying it is undefined.

Similarly, S2().x and S2().y are also defined to be
equal to zero, and here it really matters for many implementations, because
if S2().y is just a bunch of random bits, it is entirely possible
that trying to copy S2().y will yield a floating-point trap.

However, rather to my surprise, the standard does not define the value
of S3().x or S3().y, because S3 is not a POD.
It does define S3().s (by running the string constructor), but
once a structure is no longer a POD, the values of uninitialized members
are no longer guaranteed in expressions of the form T().

In my opinion, this definition is a mistake, and the committee's intention
was to zero-initialize all members that do not have an explicitly defined
constructor, whether or not the class is a POD.

151.
Terminology of zero-initialization

In 3.6.2
basic.start.init
paragraph 1 and
8.5
dcl.init
paragraphs 5 and 6, the
terms "memory" and "storage" are used in connection with
zero-initialization. This is inaccurate; it is the variables that are
zero-initialized, not the storage. (An all-zero bit pattern in the
storage may, in fact, not correspond to the representation of zero
converted to the appropriate type, and it is the latter that is being
described.)

Suggested resolution: remove the words "storage" and "memory" in
these contexts.

Proposed resolution (10/00):

Delete the words "The storage for" from the first sentence of
3.6.2
basic.start.init
paragraph 1.

[Note: Revised wording in 8.5
dcl.init
relating to this issue is also found
in issue 178.]

178.
More on value-initialization

When the Committee considered
issue 35, another context in which
value initialization might be relevant was overlooked:
mem-initializers. It would seem reasonable that if
T() as an expression invokes value initialization, that the
same syntactic construct in a mem-initializer-list would do the
same, and the usefulness of value initialization in that context is at
least as great as the standalone case.

if T is a scalar type (3.9
basic.types), the
object is set to the value of 0 (zero) converted to T;

if T is a non-union class type, each non-static data
member and each base-class subobject is zero-initialized;

if T is a union type, the object's first named data member
[Footnote: This member must not be static, by virtue
of the requirements in 9.5
class.union. end
footnote] is zero-initialized;

if T is an array type, each element is
zero-initialized;

if T is a reference type, no initialization is
performed.

To default-initialize an object of type T means:

if T is a non-POD class type (clause 9
class), the default constructor for T is called (and the
initialization is ill-formed if T has no accessible default
constructor);

if T is an array type, each element is
default-initialized;

otherwise, the object is zero-initialized.

To value-initialize an object of type T means:

if T is a class type (clause 9
class)
with a user-declared constructor (12.1
class.ctor), then
the default constructor for T is called (and the
initialization is ill-formed if T has no accessible default
constructor);

if T is a non-union class type without a user-declared
constructor, then every non-static data member and base-class
component of T is value-initialized;

if T is an array type, then each element is
value-initialized;

otherwise, the object is zero-initialized.

A program that calls for default-initialization of an entity of
reference type is ill-formed. If T is a cv-qualified type,
the cv-unqualified version of T is used for these definitions
of zero-initialization, default-initialization, and
value-initialization.

In 8.5
dcl.init paragraph 6, change "The memory
occupied by any" to "Every".

In 12.3.1
class.conv.ctor paragraph 2, insert "or
value-initialization" after the first occurrence of
"default-initialization".

In 12.6
class.init paragraph 1, replace the note by "The
object is default-initialized if there is no initializer, or
value-initialized if the initializer is ()" [i.e., replace
the non-normative note by different, normative text].

valid, which has got to be wrong. See 8.5
dcl.init paragraph
5, where there is wording that forbids default-initialization of a
reference, but not value-initialization thereof. As noted in
issue 302, if
the default constructor were required to be generated when a
value-initialization is done, that would force an error.

Proposed resolution (10/01):

Add the indicated wording to the indicated sentence in
8.5
dcl.init paragraph 5:

A program that calls for default-initialization or value-initialization
of an entity of reference type is ill-formed.

176.
Name injection and templates

There is some controversy about whether class name injection applies
to class templates. If it does apply, what is injected? Is a class
name injected or is the thing that is injected actually a template?

In general, clause 9 applies to both classes and class templates, so I
would take this to mean that class name imjection does indeed apply to
class templates. One problem with this is that clause 9 uses the
syntactic term class-name, which I would take to imply that the
inserted name is always a class. This is clearly unacceptable for
class templates as it makes the template itself unusable from with the
template. For example:

Clearly the injected name must be usable as both a class and a
class template. This kind of magic already exists in the standard.
In 14.6.1
temp.local
it says,

Within the scope of a class template, when the name of the template is
neither qualified nor followed by <, it is equivalent to
the name of the template followed by the
template-parameters enclosed in <>.

The proposal here is that we clarify that name injection does
indeed apply to class templates, and that it is the injected name that
has the special property of being usable as both a class and a
template name (as described in
14.6.1
temp.local
). This would eliminate
the need for special wording regarding the qualification of the name,
but would achieve the same result. This would also make this
"special" name available to a derived class of a class template
— something which is necessary if the benefits of class name
injection are to be made uniformly available for class templates, too.

Like normal (non-template) classes, class templates have an
injected-class-name (clause 9
class). The
injected-class-name can be used with or without a
template-argument-list. When it is used without a
template-argument-list, it is equivalent to the
injected-class-name followed by the template-parameters of the
class template enclosed in <>. When it is used with a
template-argument-list, it refers to the specified class
template specialization, which could be the current specialization or
another specialization.

Within the scope of a class template specialization or partial
specialization, when the injected-class-name is not followed by a
<, it is equivalent to the injected-class-name followed by
the template-arguments of the class template specialization or
partial specialization enclosed in <>.
[Example:

A lookup that finds an injected-class-name (10.2
class.member.lookup) can result in an ambiguity in certain cases (for
example, if it is found in more than one base class). If all of the
injected-class-names that are found refer to specializations of the
same class template, and if the name is followed by a
template-argument-list, the reference refers to the class
template itself and not a specialization thereof, and is not
ambiguous. [Example:

When the normal name of the template (i.e., the name from the
enclosing scope, not the injected-class-name) is used without a
template-argument-list, it refers to the class template itself
and not a specialization of the template. [Example:

75.
In-class initialized members must be const

A member-declarator can contain a
constant-initializer only
if it declares a static member
(9.4
class.static
)
of integral or enumeration type, see
9.4.2
class.static.data
.

But later, in the section on static class data member initialization,
9.4.2
class.static.data
paragraph 4, it says:

If a static data member is of const integral
or const enumeration
type, its declaration in the class definition can specify a
constant-initializer which shall be an integral constant expression
(5.19
expr.const
). In that case, the
member can appear in integral constant expressions within its scope.

The first paragraph should be modified to make it clear that it is not possible
to initialize a static data member in-line with a constant-initializer if that
data member is of integral (or enumeration) type, and yet not const.

80.
Class members with same name as class

Between the May '96 and September '96 working papers, the text in
9.2
class.mem
paragraph 13:

If T is the name of a class, then each of the following
shall have a name different from T:

every static data member of class T;

was changed by removing the word 'static'. Looking over the meeting
minutes from Stockholm, none of the proposals seem to include this
change, which breaks C compatibility and is not mentioned in the compatibility
annex. Was this change actually voted in by the committee?

Specifically, this breaks /usr/include/netinet/in.h under Linux,
in which "struct ip_opts" shares its name with one of its members.

190.
Layout-compatible POD-struct types

The definition of layout-compatible POD-struct types in
9.2
class.mem
paragraph 14 requires that
the two types

have the same number of members, and corresponding members (in order)
have layout-compatible types (3.9).

There does not appear to be any reason for including member functions
and static data members in this requirement. It would be more logical
to require only that the non-static data members of the two types must
match.

The characteristics of layout-compatible types are not well
described in the current wording, either. Apart from their use in
9.2
class.mem
paragraph 16 to define the
term "common initial sequence," there appears to be nothing said about
which operations are possible between objects of layout-compatible types.
For example, 3.9
basic.types
paragraphs
2-3 give certain guarantees regarding use of memcpy on
objects of the same type; it might be reasonable to assume that the
same kinds of guarantees might apply to objects of layout-compatible
types, but that is not said. Similarly,
3.10
basic.lval
paragraph 15 describes
permissible "type punning" but does not mention layout-compatible types.

Proposed resolution (10/00):

In 9.2
class.mem paragraphs 14 and 15, change all
occurrences of "members" to "nonstatic data members."

67.
Evaluation of left side of object-expression

Paragraph 2 says that "the object-expression is always evaluated" when
the class member syntax is used to refer to a static member. This
presumably should say that the object expression is evaluated if the member
access is performed, i.e., not if the overall expression is the operand
of sizeof or the unevaluated branch of ?:, ||, or &&.

48.
Definitions of unused static members

Originally, all static data members still had to be defined outside the
class whether they were used or not.

But that restriction was supposed to be lifted so that static
data members need not be defined outside the class unless they are used
in a manner which requires their definition,
in the same manner as namespace-scope
variables. In particular, if an integral/enum const static data member
is initialized within the class, and its address is never taken, we agreed
that no namespace-scope definition was required.

An expression is potentially evaluated unless either it is
the operand of the sizeof operator (5.3.3
expr.sizeof
), or it is the operand
of the typeid operator and does not designate an lvalue of polymorphic
class type (5.2.8
expr.typeid
).

is incomplete because it does not mention the use of a compile-time constant
as an array bound or template argument. It should say something like:

An expression is potentially evaluated unless it is the operand
of the sizeof operator (5.3.3
expr.sizeof
), the operand of the typeid operator,
an integral constant-expression used as an array bound or an
integral constant-expression used as a template-argument
for a non-reference template-parameter; and the expression
does not designate an lvalue of polymorphic class type (5.2.8
expr.typeid
).

An expression is potentially evaluated unless either
it is the operand of the sizeof operator (5.3.3
expr.sizeof
), or it is the
operand of the typeid operator and does not designate an lvalue
of polymorphic class type (5.2.8
expr.typeid
).

to:

An expression is potentially evaluated unless it
appears where an integral constant expression is required (see 5.19
expr.const
),
is
the operand of the sizeof operator (5.3.3
expr.sizeof
), or
is the operand of the typeid operator and the expression does
not designate an lvalue of polymorphic class type (5.2.8
expr.typeid
).

142.
Injection-related errors in access example

In the example in paragraph 3 of
11.2
class.access.base
, all the references
to B in DD::f() should be replaced by ::B.
The reason is that the class name B is private in D
and thus inaccessible in DD. (The example was probably not
updated when class name injection was added.)

161.
Access to protected nested type

When a friend or a member function of a derived class references a
protected nonstatic member of a base class, an access check applies in
addition to those described earlier in clause
11
class.access
.

This was intended to refer to nonstatic member functions and
nonstatic data members. However, a protected nested type declared
in a base class is, by
some definition of the word, a "nonstatic" member, and therefore subject
to this additional access check.

Proposed resolution (10/99):
change "protected nonstatic member" in the above to
"protected nonstatic member function or protected nonstatic data member"
to make the intent clear.

194.
Identifying constructors

According to 12.1
class.ctor
paragraph
1, the syntax used in declaring a constructor allows at most one
function-specifier. It is thus not permitted to declare a
constructor both inline and explicit. This seems
overly restrictive.

On a related note, there doesn't seem to be any explicit
prohibition against member functions with the same name as the class.
(Such a prohibition might reasonably be expected to occur in
9.2
class.mem
paragraph 13, but member
functions are not listed there.)

One possible interpretation would be that such member functions
would violate the restrictions in
3.3.6
basic.scope.class
paragraph 1,
because the class name would refer to the class at some points in the
class scope and to the member function at others. However, this seems
a bit tenuous. Is an explicit prohibition needed?

An explicit constructor constructs objects just like non-explicit
constructors, but does so only where the direct-initialization syntax
(8.5
dcl.init
) or where casts
(5.2.9
expr.static.cast
,
5.4
expr.cast
) are explicitly used...
A copy-constructor (12.8
class.copy
) is a
converting constructor. An implicitly-declared copy constructor is
not an explicit constructor; it may be called for implicit type
conversions.

This passage would appear to indicate that the call in the example is
ill-formed, since it uses neither the direct-initialization syntax nor
an explicit cast. The last sentences are especially interesting in
this regard, indicating that explicit and
non-explicit copy constructors are handled differently.

If the initialization is direct-initialization, or if it is
copy-initialization where the cv-unqualified version of the source
type is the same class as, or a derived class of, the class of the
destination... [the] applicable constructors are enumerated
(13.3.1.3
over.match.ctor
)...

The cited passage says that

The candidate functions are all the constructors of the class of the
object being initialized.

Notes from 04/01 meeting:

After the issue was accepted as a DR with the proposed
resolution to change 13.3.1.3
over.match.ctor paragraph 1
as described below, it was noticed that 12.3.1
class.conv.ctor
paragraph 3 states that:

In addition to making the proposed resolution for this issue
ineffectual, the wording of paragraph 3 also contradicts that of
paragraph 1:

A constructor declared without the function-specifierexplicit that can be called with a single parameter specifies
a conversion from the type of its first parameter to the type of its
class. Such a constructor is called a converting constructor.

These considerations led to the addition of the second point
of the proposed resolution.

When objects of class type are direct-initialized
(8.5
dcl.init), or copy-initialized
from an expression of the same or a derived class type
(8.5
dcl.init), overload
resolution selects the constructor. For direct-initialization, the
candidate functions are all the constructors of the class of the object
being initialized. For copy-initialization, the candidate functions are
all the converting constructors
(12.3.1
class.conv.ctor
) of that class.

235.
Assignment vs initialization

After the call to a constructor for class X has completed, if
a member of X is neither specified in the constructor's
mem-initializers, nor default-initialized, nor initialized
during execution of the body of the constructor, the member has
indeterminate value.

Using the term "initialized" to describe setting the value of a
member inside the body of a constructor is a misuse of the term: only
by use of a placement new expression can a member be
initialized "during the execution of the body of the
constructor."

20.
Some clarifications needed for 12.8 para 15

There are three related sub-issues in this issue, all dealing with the
elision of copy constructors as described in
12.8
class.copy
paragraph 15:

The text should make clear that the requirement that the copy constructor
be accessible and unambiguous is not relaxed in cases where a call to a
copy constructor is elided.

It is not clear from the text that the two optimizations described can
be applied transitively, and, if so, the implications for the order of
destruction are not spelled out.

The text should exclude applying the function-return optimization if the
expression names a static or volatile local object.

Analysis

After discussion in Santa Cruz, the core group decided that sub-issue
#1 required no change; the necessity of an accessible and unambiguous copy
constructor is made clear in
12.2
class.temporary
paragraph 1 and need not be repeated in this text. The remaining two sub-issues
appear to be valid criticisms and should be addressed.

Proposed Resolution (10/99):

[Note: a small portion of this wording is superseded by the
resolution of issue 185.]

The paragraph in question should be rewritten as follows.
In addition,
references to this section should be added to the index under "temporary,
elimination of," "elimination of temporary," and "copy, constructor elision."

When certain criteria are met, an implementation is allowed to omit
the copy construction of a class object, even if the copy constructor
and/or destructor
for the object have side effects. In such cases, the implementation treats
the source and target of the omitted copy operation as simply two different
ways of referring to the same object, and the destruction of that object
occurs at the later of the times when the two objects would have been destroyed
without the optimization [footnote: Because only one object is destroyed
instead of two, and one copy constructor is not executed, there is still
one object destroyed for each one constructed. end footnote].
This elision
of copy operations is permitted in the following circumstances (which may
be combined to eliminate multiple copies):

in a return statement in a function with a class return type, where the
expression is the name of a non-volatile automatic object with the same
cv-unqualified type as the function return type, the copy operation can
be omitted by constructing the automatic object directly into the function's
return value

when a temporary class object
(12.2
class.temporary
)
would be copied to a class object with the same cv-unqualified type,
the copy operation can be omitted by constructing the temporary object
directly into the target of the omitted copy

Here the criteria for elision can be combined to eliminate two calls to
the copy constructor of class Thing: the copying of the local
automatic object t into the temporary object for the return value
of function f() and the copying of that temporary object into
object t2. Effectively, the construction of the local object t
can be viewed as directly initializing the global object t2, and
that object's destruction will occur at program exit.
—end example]

51.
Overloading and user-defined conversions

In 13.3.3
over.match.best
paragraph 1,
bullet 4 of the second set of bullets, there is a cross-reference
to 8.5
dcl.init
and
13.3.1.5
over.match.conv
. I believe it should also
reference 13.3.1.6
over.match.ref
.
I think the phrase "initialization by user-defined
conversion" was intended to refer to all initializations using user-defined
conversions, and not just the case in
13.3.1.5
over.match.conv
. Referring to only
13.3.1.5
over.match.conv
suggests a narrower
meaning of the phrase.

13.3.1.4
over.match.copy
,
although it does deal with initialization by user-defined
conversion, does not need to be referenced because it deals with class
—> class cases, and therefore there are no standard conversions involved
that could be compared.

84.
Overloading and conversion loophole used by auto_ptr

By the letter of the standard, the conversions required to
make auto_ptr work should be accepted.

However, there's good reason to wonder if there isn't a bug in the standard
here. Here's the issue: line 16 in the example below comes down to

copy-initialize an auto_ptr<Base> from
an auto_ptr<Derived> rvalue

To do that, we first look to see whether we can convert
an auto_ptr<Derived>
to an auto_ptr<Base>, by enumerating the
constructors of auto_ptr<Base>
and the conversion functions of auto_ptr<Derived>.
There's a single possible way to do the conversion, namely the conversion
function

auto_ptr<Derived>::operator auto_ptr<Base>()

(generated from the template). (The constructor
auto_ptr<Base>(auto_ptr_ref<Base>)
doesn't work because it requires a user-defined conversion on the
argument.)

So far, so good. Now, we do the copy step:

direct-initialize an auto_ptr<Base>
from an auto_ptr<Base>
rvalue

This, as we've gone to great lengths to set up, is done by calling
the conversion function

auto_ptr<Base>::operator auto_ptr_ref<Base>()

(generated from the template), and then the constructor

auto_ptr<Base>(auto_ptr_ref<Base>)

(generated from the template).

The problem with this interpretation is that it violates the long-standing
common-law rule that only a single user-defined conversion will be called
to do an implicit conversion. I find that pretty disturbing.
(In fact, the full operation involves two conversion functions and two
constructors, but "copy" constructors are generally considered not to be
conversions.)

The direct-initialization second step of a copy-initialization was intended
to be a simple copy — you've made a temporary, and now you use a copy
constructor to copy it. Because it is defined in terms of direct
initialization, however, it can exploit the loophole that auto_ptr
is based on.

To switch to personal opinion for a second, I think it's bad enough
that auto_ptr has to exploit a really arcane loophole of overload resolution,
but in this case it seems like it's exploiting a loophole on a loophole.

It seems clear to me that the result of this direct initilization
must be the second standard conversion sequence in a user defined
conversion sequence. Otherwise the resulting conversion sequence is
not an implicit conversion sequence. By the letter of the standard,
the sequence of conversions making up a copy-initialization must be an
implicit conversion sequence.

An implicit conversion sequence is a sequence of conversions
used to convert an argument in a function call to the type of the
corresponding parameter of the function being called. The sequence of
conversions is an implicit conversion as defined in clause
4
conv, which means it is governed by the rules for
initialization of an object or reference by a single expression
(8.5
dcl.init, 8.5.3
dcl.init.ref).

The initialization that occurs in argument passing ...
is called copy-initialization and is equivalent to the form

T x = a;

For me, these sentences imply that all sequences of conversions
permitted on a function argument must be valid implicit conversion
sequences.

The 'loophole' can be closed by adding a sentence (or note) to the
section describing the 'direct initialization second step of a copy
initialization' stating that the copy initialization is ill-formed if
the conversion sequence resulting from the direct initialization is
not a standard conversion sequence.

In the context of an initialization by user-defined conversion (i.e.,
when considering the argument of a user-defined conversion function;
see 13.3.1.4
over.match.copy, 13.3.1.5
over.match.conv),
only standard conversion sequences and
ellipsis conversion sequences are allowed.

to

A well-formed implicit conversion
sequence is one of the following forms:

However, when considering the argument of a user-defined conversion
function that is a candidate by 13.3.1.3
over.match.ctor
when invoked for the copying
of the temporary in the second step of a class copy-initialization, or
by 13.3.1.4
over.match.copy, 13.3.1.5
over.match.conv,
or 13.3.1.6
over.match.ref in all cases, only standard
conversion sequences and ellipsis conversion sequences are
allowed.

83.
Overloading and deprecated conversion of string literal

S1 and S2 differ only in their
qualification conversion and
yield similar types T1 and T2
(4.4
conv.qual
),
respectively, and the cv-qualification signature of type T1
is a proper
subset of the cv-qualification signature of type T2,
[Example:

This does not work right with respect to the deprecated conversion from
string literal to "char *". Consider

void f(char *);
void f(const char *);
f("abc");

The two conversion sequences differ only in their qualification conversions,
and the destination types are similar. The cv-qualification signature of
"char *", is a proper subset of the cv-qualification signature
of "const char *", so f(char *) is chosen, which is wrong.
The rule should be like the one for conversion to bool — the deprecated
conversion should be worse than another exact match that is not the deprecated
conversion.

S1 and S2 differ only in their qualification
conversion and yield similar types T1 and T2
(4.4
conv.qual
), respectively, and the
cv-qualification signature of type T1 is a proper subset of
the cv-qualification signature of type T2.

to

S1 and S2 differ only in their qualification
conversion and yield similar types T1 and T2
(4.4
conv.qual
), respectively, and the
cv-qualification signature of type T1 is a proper subset of
the cv-qualification signature of type T2, and S1 is
not the deprecated string literal array-to-pointer conversion
(4.2
conv.array
).

202.
Use of overloaded function name

13.4
over.over
paragraph 1 contains a
supposedly exhaustive list of contexts in which the name of an
overloaded function can be used without an argument list ("...shall
not be used without arguments in contexts other than those listed").
However, 14.3.2
temp.arg.nontype
paragraph 5,
bullet 4 gives another context: as a template nontype argument.

If the name is a function template, template argument deduction is
done (14.8.2.2
temp.deduct.funcaddr), and if the argument
deduction succeeds, the deduced template arguments are used to
generate a single template function, which is added to the set of
overloaded functions considered.

It is not clear whether this formulation allows explicit
specification of non-deduced template arguments. For instance,

Suggested resolution: Clarify 13.4
over.over
paragraph 2 to allow both deduced and explicitly-specified
template arguments to be used to determine the function template
specialization to be added to the overload set.

32.
Clarification of explicit instantiation of non-exported templates

A non-exported template that is neither explicitly specialized
nor explicitly instantiated must be defined in every translation
unit in which it is implicitly instantiated (14.7.1
temp.inst
) or explicitly
instantiated (14.7.2
temp.explicit
); no diagnostic is required.

Shouldn't the first underlined phrase be omitted to avoid conflict with
the second underlined phrase?

From John Spicer:

The first "explicitly instantiated" is intended to mean "explicitly instantiated
in some other translation unit".

A non-exported template that is neither explicitly specialized
nor explicitly instantiated must be defined in every translation unit in
which it is implicitly instantiated (14.7.1
temp.inst
) or explicitly instantiated
(14.7.2
temp.explicit
); no diagnostic is required.

to:

A non-exported template must be defined in every translation
unit in which it is implicitly instantiated (14.7.1
temp.inst
), unless the corresponding
specialization is explicitly instantiated (14.7.2
temp.explicit
) in some translation
unit; no diagnostic is required. [Note: See also 14.7.2
temp.explicit
]

105.
Meaning of "template function"

The phrase "template function" is sometimes used to refer to a
template (e.g., in 14
temp
paragraph 8)
and sometimes to refer to a function generated from a template (e.g.,
13.4
over.over
paragraph 4).

Suggested Resolution:

The phrase should mean "a function generated from a template"
(or might perhaps include explicit specializations).